What would cause matted hair with a cat?

A few weeks ago my short-haired cat started getting matted fur. I brushed him and used a “comb” (actually a multi-bladed curved bladed tool that looks pretty wicked) to get the mats out. I was kind of surprised the short-haired one was getting them and not the long-haired one.

Now the long-haired one is getting them. I brushed him out last night.

Anyway, I’ve never seen either of these cats get matted hair before, and they used to roam around outside all the time. But now they’re solely indoor cats.

What could be causing this? Could it be diet? I noticed that the food I usually give them changed something (I’m not sure what, but the bag says it’s different). Or could it be that because they don’t get outside and caught in trees, bushes, etc, that the hair is building up?

Is it a psychological thing, maybe? They used to have a huge house to roam around in and now we’re packed like sardines in this tiny apartment.

Any ideas?