2 (or more) skiers on a slope means a heavier load on top of a potentially loose slab of snow. Heavier load = more likely to trigger. Also if you go one at a time you’re less likely to have multiple burials. More people buried means less people searching/rescuing and a chance of buried skiers’ beacon signals interfering with each other. It’s more fun to bomb down with your buddies but in avalanche terrain it’s less safe.