I think it’s pebbles. Run around, foot gets caught on a pebble. Music plays “Dumb ways to die!”

The Crucible was made by the same people who made Halo, and it used to feel fantastic to play. The issue is they all but abandoned the mode in 2020, when they deleted half the maps and moved their entire PvP group on to other projects. Turns out when you do nothing meaningful to a game mode, that game mode gets worse and will go into a death spiral.