Well, at the risk of sounding a bit dim, my little 9 month old terrier bitch appears to be humping things. She does it on her cushion and also tried it on our other dog (spayed male). I guess she would have maybe been coming into season and this is her way of feeling a little frisky. Is it normal? Hope you don't all think I'm nuts for asking this.
lots of dogs/bitches do this so don't worry - we used to have one that would drag towel, cushion whatever out and give a demonstration particularly when we had visitors -we'd never acknowledged the behaviour so can't think what prompted her to think we were interested. may settle down a bit when she's spayed.
It's normally a dominance matter, though not serious. Sometimes it's sheer impertinence! One bitch here humps her own mother.The mum ignores this, with great maternal forbearance, most of the time, but every now again gets fed up and turns and tells her daughter off! They've been going through this performance for years not, happily, now every day but still often enough !