This is obviously an Americanism. Australians are probably using butt more often than they used to but I would say that bum is still top frequency. This name has been chosen by Paco Underhill, an American described as the founder of market research, for the negative experience shoppers may have when the path around the shop, supermarket, etc., has not been carefully planned, with the result that they end up being crowded by other people. Women, in particular, it seems, do not like to have their bottoms brushed by other shoppers in close proximity. They tend to move away as fast as possible, and thus the opportunity to sell them the can of tomatoes they were considering purchasing is lost forever.
By the way, since Underhill created the study of shopping, innocents abroad in the supermarket need to be on the lookout for field researchers or trackers. Have you noticed someone standing at the entrance, and then following a shopper while making notes, quickly darting away when they think they might be spotted and then returning stealthily to renew their observations? Your next butt-brush may be recorded in the interests of science.
The butt-brush effect has been extended to cover any kind of jostling that creates negative feelings that inhibit purchasing.
It is possible that I may have alerted people to the existence of trackers in supermarkets on the watch for butt-brush effects and this may increase the frequency of the word, but I suspect it will remain a bit of marketing jargon.