It occurs to me that another reason for Trump to talk about rigged elections and not accepting the results of the election is so he can call Hillary a hypocrite when Russian hackers deliver him the election and Hillary disputes it.
That sounds like a far-fetched scenario, but not impossible. We know that voting machines can be hacked to produce a total count that doesn't match the actual votes cast. We know that manual recounts are done (in most places) only when the reported vote total is very close, and even then political appointees may control how thorough the recount is. We know that some jurisdictions in the U.S. use electronic-only voting machines that make a manual recount impossible.
So if I were a security-cracker who wanted to shift the outcome of a U.S. election, I would pick a bunch of states where polling indicated my candidate was losing narrowly. On each voting machine, if a vote is cast for other than my preferred candidate, with 10% probability I record it instead as a vote for my preferred candidate. If the actual vote is 50% Clinton, 45% Trump, that hack is enough to reverse it to 50% Trump, 45% Clinton: a large enough margin to avoid a recount, but not so large as to be completely implausible.