I guess they get thrown a long way but luckily land on their feet
With regards to balancing tramplers... when there's only a few on the board, they tend to get surrounded and pulled down quickly. When there's a lot, they tend to form a "wall" that is much more powerful. In itself it's a good thing, but it seems a bit too strong and needs to get toned down somehow.
If the repel idea isn't liked, the reducing quantity per squad (via making leadership take size into account) would make it harder to accumulate large masses of tramplers. (Leadership 40 = 80 halflings, 40 humans, or 6 elephants)
Reducing elephants per squad would make it somewhat harder to mass huge hordes, and greatly increase the likelihood of a rout by at least some of them.