They become disabled when the owner no longer wishes to run it. A command is run to remove the witness from the rotation. A lot of the old miners are on the witness list but haven't been online in a long time. Take a look at @drakos' https://steemian.info/witnesses for the list. All the ones in the red are disabled. If "block age" is very low and they're red that means the owner took it offline for maintenance.

In the case of pumpkin, it's been off since HF 16. You can also see that it supports the registration fee of 100 steem and some other parameters. Also that 174 accounts still vote for it despite it having long been disabled.