Strange bad form can hit your team if you have put a superb quality player on the starting lineup compared to your captain. Usually, this only happens if you buy scouts or your captain is under 6*, and is easily identified as your captain suddenly have form drop to 5 or less.

The captain need to have enough skill himself in order to command the team.

This functionality is extremely annoying, I hate to change captain but when this happen I found no other choice.
Failure to respond to such team crisis can cost you many games.