What triggered an attempted coup while Erdogan was holidaying?

Despite Erdoğan's crackdown on freedoms, Turkish people chose to take to streets to support him

Following a Friday night of unrest as a result of the attempted military coup in Turkey, which appears to have failed, several questions have been raised necessitating a comprehensive explanation of the motives behind the attempt by some dissenters—comprised of military personnel.
The attempted military coup prompted supporters of the Turkish president Recep Tayipp Erdoğan, who was out of the country on holiday, to take to the streets and intervene in any attempts of military personnel to take over strategic places in Turkey.

On Friday night, the plotters of the coup reportedly took control of Turkish state-owned television and aired a statement, in which they said that the army is in charge of the country and the treaties and relations with foreign countries are still in effect.
According to the people who claimed to speak for the Turkish military in the statement, they said that the motive for the coup was Erdoğan's negligence of Turkey's democratic secular traditions.

Reasons for the attempted coup
Etyan Mahcupyan, a pro-government columnist told Daily News Egypt that this is not a proper military coup as the military opposed such a movement, but rather a particular number of military officers attempting to make a coup.
"The Fethullah Gulen supporters are behind the attempted coup, even though he denied any connection to what happened, but not all of them are following what he says," Mahcupyan said.
He added that the attempted coup happened because these military officers weren't going to receive any promotions in August, which is the annual timing for military promotions and position changes, saying that they are Gulenists and cannot be trusted inside the military system, and would therefore get no more than a desk position.
Despite Erdoğan's many achievements throughout his years as a Turkish leader, beginning in 2002 when he was prime minister, he couldn't manage to escape criticism.
The mega, national projects and the significant forward jumps that the economy took in his era haven't interceded for his autocratic regime that aimed to silence dissent in Turkey.
Erdoğan has vigorously attempted to silence all opposition figures. Press freedom groups claimed that around 900 journalists lost their jobs as a result of criticising Erdoğan's policies. Everyone is afraid to question him publicly.
Since 2014, journalists have been arrested and charged with espionage. The opposition newspaper Zaman has been seized along with other newspapers and TV channels that were opposing the government's policies. The crackdown has also led to the harassment and deportation of foreign reporters.
In his plan to create a one-voice nation, Erdoğan and his Justice and Development Party have grown to create an authoritarian regime that believes the only suitable place for dissenters is prison.

In its 2015 World Press Freedom index, Reporters Without Borders ranked Turkey 149 out of 180 countries in press freedom.
Another reason for the attempted coup was the controversial Turkish anti-terrorism law. This law enables the government to prosecute journalists and government critics, basically any opposition figure. Despite the pressure by European leaders on Turkey to change its controversial law, the Turkish government insists that the draconian laws are used to counter the threat of Kurdish and the Islamic State terrorism.
In continuity to the crackdown on freedoms by Erdoğan's regime, in May, the parliament ratified a bill that stipulates stripping some politicians from immunity. Members of the pro-Kurdish People's Democratic Party said it targeted them particularly, as it removed immunity from a total of 138 deputies in the parliament: the same number of members who were facing criminal charges.
Another reason why a military coup was attempted in Turkey was the continuous accusations of corruption against Erdoğan, despite his denial of wrongdoing. He always blames his former ally Fethullah Gulen, who is also his current bitter enemy, of trying to set him up to topple his regime.

Erdoğan explicitly accused Gulen on Friday of leading the attempted coup; however, Gulen announced that he opposes such a move, going so far as to condemn the coup.
Gulen, a Turkish cleric, was a former ally to Erdoğan and helped him rise to power; however, he now lives in Pennsylvania, United States and is at the top of the wanted list.

Turkey's history with military coups
Despite the severe crackdown on opposition in Turkey, when the attempted coup occurred on Friday, tens of thousands took to the streets in opposition of the attempted coup, not particularly because they are encouraging the severe crackdown on freedoms, but rather because they have a relatively long history with coups that didn't end in their favour. In the coup that occurred in 1980, though it brought more stability, thousands were killed while dozens were executed and tortured.