Devils advocate: You are responding to a victim down, surrounded by a crowd of 15-20. You know violence has already occurred and now you are surrounded/outnumbered with no immediate way to differentiate friend from foe. You tell the crowd to back off, but some keep pushing in on you...while you are trying to secure the scene so medics can get in.

You tell them multiple times to back up, they still remain/move in. You tell them they will be arrested for obstructing an officer in performance of their duties. They continue to move in.

You cuff them and sit them down so you can deal with the situation at hand. Charge them. The DA later chooses to drop charges.

It does not mean the person arrested was innocent. It also does not mean they were guilty. It certainly isn't false arrest.