Well an f/t hopper should be a fine size and not too big so I don't think that's the issue for the spitting out behaviour. If you are sure you're fully thawing and warming up the prey correctly, then I'm betting this is just a snake that got spooked while feeding. I've had a full sized adult female do this when I accidentally bumped her enclosure with my hip while she was in the process of swallowing. They feel very vulnerable then, they can't fight or flee easily with a mouth full of rat/mouse, so their only option if they feel threatened is to spit it back (not a regurge as it was never fully swallowed).

We've had a couple of snakes here that must have their enclosures covered while they eat so I keep some old dark blue sheets around. Leave them exposed, they will refuse to eat....cover them up temporarily, they hit the prey faster than the eye can see.