Possibly in extreme response to the overcrowding there. Repot it in its own pot, taking care to remove the damaged leaves (or not), and it should be fine.

Thanks, never seen this before. They have been growing in that pot for 3-4 years. I will separate them to their own pot this week!

I think it's a survival thing for Sans to do this - not uncommon when the leaves are not getting the nutrients they need fully, and therefore shut down to keep the root area alive for as long as possible (months, anyway).

Looks like sun burn and big change in environment. Cut off the bad sections so not to spread rot. Like Cactusmcharris said, put it in it's own pot.