If, after 30 days, you haven't chosen and selected a Most Helpful opinion, one will automatically be chosen for you. Neither the Question Asker nor the user awarded Most Helpful will get Xper points for these automatic selections. It's chosen based on engagement: votes, comments, length, time, xper level, gender, among other criteria. As long as you're giving helpful opinions, we'll find them!
This is to encourage everyone to choose a MHO on their own, making sure people get the credit they deserve.
I got a MH as an auto select once and it was a troll reply by me (there were many genuine /helpful answers as well). So I think its picked randomly by some stupid algorithm as they clearly didn't even read it.
Because enough people pissed and moaned about it on the old site, as if not selecting a BA is a life or death conundrum