It is utterly absurd to put DYs reason down to immaturity. It comes from the most basic human need, the need to feel important. The girl who boasts about her husband in kollel is no more immature than the girl who boasts about how smart her baby is for saying NO to carrots, or some other stupidity. No more immature than the grandma on the bus who forces you to look at her beautiful grandchildren (are those your grandchildren or their dogs, lady?). No more immature than popa trying to make everyone laugh (okay, maybe that actually is immature). No more immature than telling the gabbai to call you up as harav or ben harav instead of Reb.
The question then becomes how did saying my husband is in kollel become something that makes a girl feel important? The answer is obvious.