This doesn’t sound at all like true diarrhea. It sounds like typical ISC, intermittent soft cecotropes aka poopy butt/sticky poop. Some buns are prone. My Bam is. If he gets too much of anything (except hay) he gets poopy butt. But he can also get poopy but “for no reason” – meaning I havent been able to find the reason. This has been going on for as long as I’ve had him, regardless of dietary regimens, so I think he has a proneness. I give him a probiotic powder when it happens. That seems to clear it up, but maybe it would’ve anyway, that’s not possible to say.
A diet high in fiber is the standard recommendation. Here is an article about ISC, intermittent soft cecotropes:
http://rabbit.org/intermittent-soft-cecotropes-in-rabbits/
This article is good too:
http://rabbit.org/the-mystery-of-rabbit-poop/
ETA: Just yesterday there was a big blob of poop stuck to his rear, I felt it when I lifted him up for our routine morning cuddle. I removed the blob of and put it in the litter box, thinking I’d throw it out after I’d wiped his butt. But I forgot, and as soon as I put him back down, he jumped into the box and gobbled up the blob. Yum-yum-yum. 
