but I'll give it a shot...
Hu fell off the tree past Wil, and even though there was such a small moment of time in which Wil could have offered help to a falling Hu, Hu took offense to the lack of assistance Wil offered during the moment in which Hu was falling past him.
It's a silly joke at best, with very little humor value without proper context. However, as a farmer with experience in feuds, Rand had the proper context for this joke to have high humor value.