I think putting this result side-by-side with the Brexit vote sums up pretty well where electorates in at least Britain and the US are these days: not strongly favoring any particular party or faction, just dissatisfied with the status quo and looking for some kind of answer.
Here's a slightly different way to see this election:
Something beats nothing. And in the end, May was nothing.People – many people, that is, not a majority but enough to turn elections – are looking for a vision. They are not impressed by managerial competence or fidelity to old ideas. They want to hear that somebody is going to change things in a way that helps them. By defiantly promising only more of the same, May messed up big-time.