Put as a question as I can’t claim to know enough to be sure. But some are castigating Tsipras for being irresponsible and subjecting Greece to unwanted and unnecessary uncertainty so soon after concluding the 3rd bail-out agreement, and receiving the first tranche of money [paid straight to the ECB].
An alternative narrative is that once the agreement was signed, it was a foregone conclusion that the Left Platform group and supporters would ultimately withdraw all support. That would force him to rely on the cooperation of too many opposition MPs, who would extract a political price that might undermine him, and perhaps the agreement itself. [Really, what else is he now, except the agreement?] Indeed, they would be looking for the first chance to sink him and his Syriza rump.
Given that foregone conclusion, better to call the election soon, to give LP as short a time as possible to organise their own Party. Obviously he calculates that he will swap LP MPs for almost as many names that he has hand-picked for his new party list, and who will owe their new career to him, and thus be bound into his personal project of implementing the agreement successfully. If an election has to happen to secure Tsipras’ hand, and thus ultimately the fate of the agreement [which lies in demonstrating cooperation over implementation] what benefit is there to waiting?
It might look odd, since ultimately Tsipras decide that the Referendum’s ‘No’ meant ‘Yes’. But he has presumably been looking at the same polls as everyone else, and sees that no-one else can capitalise on that abrupt change in strategy, or facing up to reality, however one sees it.