Jimmy freaked out while i cleaned his pen

[JimmythePig]

I have a gated playpen that I attach to Jimmy's pen. I use it all the time and this is actually where he spends a lot of his time, running back to the pen for a drink or a nap, he is pretty much in the middle of the living room. When I clean his cage I close off the playpen, top off the hay and put his water and a tunnel in there. Usually it's fine.
This time he freaked out. I had everything removed from the pen, and Jimmy just went crazy. He pushed his way out of the playpen and started to bite the rails on his pen. His pen is on the floor so I opened it and let him in. He seemed upset by the empty pen and ran around to the spots where his hideaways and tunnels usually are.
I managed to reassemble the pen with him in it.
I don't get it. Other than that he is doing really well. Eating, drinking, popping and peeing with no issues. He is getting very comfortable with me and shows no signs of being afraid.
Jimmy is a lone boar piggy I am fostering. Is this a bad sign for him getting along with a companion?Is there anything I can do to lessen his stress and prepare him for a potential bond? And why did he suddenly freak out? I don't want him to stress at the next cage cleaning.

 

[Piggies&buns]

This will have no bearing on how he gets on with a companion.
It sounds as if something scared him - they have more sensitive hearing to us so perhaps he heard something that spooked him, perhaps being in the pen with no hideys scared him as he had no where to go.

 

[JimmythePig]

Thank you. I thought he may have been acting territorial because I had taken over his pen!