Cats like pulling things down off of other things. It entertains them.

Maybe they don't think your towels match the decor.

Three possible reasons:

1- They're into your smell and trying to rub themselves on something that smells like you,

2- they're trying to get your attention,

3- if the litter is also in the bathroom, they could be cleaning their paws on the towel. I've seen cats scratch everything in the room after using the litter.

From the way you phrased the question my money is on #2, but you know better. It might be time to get a higher towel bar or close the bathroom door

Asserting authority over it's domain.

Pure malice.

I have a small dog who likes to pull anything down that he can. Blankets off of the couch, towels off of the bar, sweatshirts off of the chair. He seems pretty chuffed with himself he proudly sits next to his handiwork. My wife and I think he just likes doing it because it's all he really can do. It's harmless and he seems to enjoy it.