There are multiple points at which something requires resolution, and their could even be more depending on how complicated the narration becomes. One s a roll against a static condition, the slippery floor slide, and the other is an opposed roll against an active opponent, grabbing someone's leg that doesn't want to be grabbed. By claiming it is a penalty, you suggest there is an adversarial relationship between player and DM, which isn't the case. Arguing at the table that the DM is trying to penalize a player would certainly bring such a game to a crawl. It's not personal, it is just an adjudication of the situation based on the factors presented and utilizing only two types, one each, of standard resolution mechanic. Adding in a plus ten for whatever reason seems like extra math for its own sake. Why not just set the DC ten lower and do a straight roll? Do Spinal Tap's amps really need to go up to eleven?