I'm just curious why this thread was deleted twice. I mean, what's the motivation and how this happened? (Someone proposed it for deletion or something else happened?)
For the "how": in this case 3 users with 10k+ voted to delete the post, you can see who in a banner similar to the close banner.
Closed questions (after two days) become eligible for deletion, by votes of 10k+ users, which can be cast just like close votes; go to a question that is closed since a while to see the "delete" button.
There is also a specific section of the 10k+ tools dedicated to this, see https://math.stackexchange.com/tools "delete" that provides a way for some coordination of such votes.
For many details on deletion see How does deleting work? What can cause a post to be deleted, and what does that actually mean? What are the criteria for deletion?
For the "why": I cannot know for a fact, but the question lacks all context and motivation, and such questions are sometimes deleted. If you disagree with a specific deletion and want it undone a way is to post in http://meta.math.stackexchange.com/questions/19042/requests-for-reopen-undeletion-votes-etc-volume-01-2015-current-versio