From Bet Labs.....................
Since 2005, underdogs have gone 2,419-3,273 (42.5%) in September according to Bet Labs Sports. This might seem like a losing record, but because of the plus money payouts you are actually up +6.45 units. This is one of the only months (aside from April and October) where blindly betting underdogs is profitable.
The key is ensuring they also have line movement in their favor (which means likely sharp support). If you look at dogs with at least 1-cent of line movement in their favor in September they improve to 44.7% with +78.39 units won.
The key here is that public bettors leave the MLB market in favor of NFL and NCAAF. As a result, almost all line moves in September are caused by sharps. So tailing the sharp side becomes more popular than usual.