Stomping Tantrum counts all sorts of failures except striking at a target that used Protect. This includes type immunities, ability immunities, missing due to accuracy, or anything else that makes a move end early with a message (like using Rain Dance when it's already raining).
If you're using a pledge combo (two different moves from Grass Pledge, Fire Pledge, and Water Pledge used by the same team on the same turn), the faster Pokemon is also treated as failing their move in order to exit out and hand off to the slower one to perform a powered-up move. If the faster Pokemon then uses Stomping Tantrum next turn, it will get doubled power.