Human are persistence hunters. Well-sculpted male buns imply great hunting prowess (and thrusting potential).
Male butts can also be a symbol of vulnerability, particularly for women into girly guys. Sexualizing a typically submissive body part might make women feel safer or more in control. Especially since it's an erotic body part that doesn't have the historic and social connotations of Penis-in-Vagina sex; male butts have nothing to do with a 'woman's role' in the bedroom, and so imply a more fun, baggage-free sexual encounter.
Similarly, it can be a way for women to sexually objectify men in the exact way that men objectify women. Men show their chests freely and have no vaginas, but butts are equal-opportunity targets of embarrassment.
Plus, butts are squishy. People will sexualize any squishy body part.