Rag, here is how she justified it.

"So we need a general election and we need one now. We have at this moment a one-off chance to get this done while the European Union agrees its negotiating position and before the detailed talks begin."
"I have only recently and reluctantly come to this conclusion. Since I became prime minister I've said there should be no election until 2020, but now I have concluded that the only way to guarantee certainty and security for the years ahead is to hold this election and seek your support for the decisions we must take."

Jeremy Corbyn supported the move and a two thirds majority of MPs was required to bring the date forward.