The TL;DR of why this happened is there was expected to be a vote of no confidence against him. People of the UK were going to make him go through hell and trials and such and he was expected to leave anyways, so he resigned preemptively.

This is a win for the people though, it's good he knew he was going to lose anyways and not have us do the song and dance. What happens now though is a little worrying, the sudden resignation of him and 50+ people in the government means it's a little confusing who's going to rule now.