This dominance behavior. She may have done something as simple as mot grooming him long enough that made him feel a need to exert himself. My bonded pair have been been bonded since birth (they will be a year old in a few weeks). I’m pretty sure Moshi is the dominant bun, but every once in a while, Marlee will decide to hump Moshi (she’s definitely smaller than he is). She will usually nip his butt if he annoys her (he’s a bit clueless — I’ve actually seen him flop on top of her in the litterbox as well as try to get her to groom him while she was peeing!) bit if that doesn’t work she might hump him. He’ll usually just hop away, she’ll follow, he’ll stick his head under hers, and all is forgiven. Occasonally he won’t hop away but will turn around and hump her back. I’ve even seen them go so far as brief circling, but one or the other will always start grooming the other one. With mine, this only happens very rarely and only in the common play area (we have another bun).
Since yours have only been together for two months, you could watch to see how they work it out and do a little stress bonding if it keeps up.