She didn't jump, she was pushed. The resignation happened immediately after a meeting with the chairman of the party's "1922 Committee" - that's the body that (to greatly simplify) gets to make the rules about how they get rid of their leaders.

This is standard in parliamentary democracies; when the PM knows that they're about to be removed, they'll leave. If she hadn't resigned, she'd be going anyway.

Even in non-parliamentary democracies, see Richard Nixon; he made a lot more fuss, of course, but he did, in the end, resign.