Just imagine if the toys were locked in the box.
The toys weren't played with for years, and were kept in a box. When there were no humans present, they got out of the box and did things.
But what if the box was locked? What if they were forced to spend years squashed together in a dark box, unable to move? They might not even be able to talk, because they wouldn't be able to know whether or not there were humans in the room.
Can toys suffer from mental illness? I see no reason why they can't, they're exactly like humans. And any human in that situation would go crazy. Just imagine if Bonnie ended up playing with mentally ill people without realising. Pretty terrifying.