What does "crowding mushrooms" mean?
It's not the mushrooms that are important, it's the "crowding" -- basically, you don't want so much food in the pan at once that the bits are packed tightly, or in more than one layer.

This is especially important for items that give off a lot of water as they cook (like mushrooms), or that you're trying to get to crisp up, as you want to leave space for the moisture to evaporate and escape without causing the food to steam.