(First of all, let me mention that I did not upvote the questions mentioned.)
Two of the questions you mentioned were from new users (those with 1 reputation and asking a question for the first time in MathOverflow). As you probably know, StackExchange currently has Winter Bash 2014. There is a secret hat called Warm Welcome (https://meta.stackexchange.com/a/245397/177408) and it is earned by upvoting a new user's first post.
One possibility is that some MO users don't know that MO is not participating in Winter Bash 2014 and they upvote these questions in an attempt to get this hat.