You can see on the video below people exhibiting anger toward the government police, perhaps some thinking that the government had targeted them. The other possibility is ISIS seeking to ratchet up government/Kurdish tensions prior to the election in November.
Via Yahoo:
There was no immediate claim of responsibility but Prime Minister Ahmet Davutoglu said there were “strong signs” that the two explosions — which struck 50 meters (yards) apart just after 10 a.m. — were suicide bombings. He suggested that Kurdish rebels or Islamic State group militants were to blame.
The two explosions occurred seconds apart outside the capital’s main train station as hundreds of opposition supporters and Kurdish activists gathered for the peace rally organized by Turkey’s public workers’ union and other groups. The protesters planned to call for increased democracy in Turkey and an end to the renewed violence between Kurdish rebels and Turkish security forces.
The attacks Saturday came at a tense time for Turkey, a NATO member that borders war-torn Syria, hosts more refugees than any other nation in the world and has seen renewed fighting with Kurdish rebels that has left hundreds dead in the last few months.
Many people at the rally had been anticipating that the rebels of the Kurdistan Workers’ Party, or PKK, would declare a temporary cease-fire — which it did hours after the bombing — to ensure that Turkey’s Nov. 1 election would be held in a safe environment.
Here’s video immediately after the bombing. WARNING! It is very graphic, there are wounded and dead, so we are placing it beneath the fold, it is not for the squeamish.