Maybe the FG faithful threatened a vote of no confidence, so he jumped before he was pushed - he hasn't exactly been popular the last few months, and they're now aware of bad polling numbers and an upcoming election. Also the referendum disaster was the final nail in the coffin. With Leo at the helm the voters see more of the same and don't want it.
I see he made sure to still get his US junket in before he scarpered.
I've also heard rumours he's not very popular in the EU, hasn't made many allies and so not sure if he will try and go for a cushy EU job.