Sometimes this is an infection like the other poster suggested. But often times it's a behavioral thing. Sometimes they are marking territory if there are other cats in the house. But sometimes they develop fetishes.
I have a cat that will pee on anything that is on the floor and wet. This means we can not have a towel or rug on the floor outside the shower. So we got rid of the rug and we pick up our towels immediately. We can not keep wet laundry in a basket in the hallway or the cat will pee in it. Oddly enough, I find this behavior interesting in that we have no business leaving this stuff lying around. Is it possible in some strange way the cats like the house to be clean just so? When my house is picked up and we don't have laundry laying around we have zero problems.
I also have a theory about the blankets and clothing on the floor. Dirty clothes and towels we have dried ourselves with have bodily odors and fluids on them. It's in such a trace amount we don't notice it, but cats do. It's possible that in their mind that stuff is wet, it stinks already so it should be peed on.
Also, sometimes they don't like their litter, where it's at or that they are sharing it. I make it a rule to have litter in at least 3-4 places in the house and to have it clean all the time. We have tried various kinds until we settled on the crystal cat litter that Walmart sells. It's the only litter of it's kind that is affordable. It keeps the urine from smelling just so long as we change it weekly. Since we have 4 boxes in the house, none of them get too bad before we change them.
Sometimes the animal is mad for another reason like they don't like the food or that they feel neglected or something in their lives has changed.
With a cat like this you just have to be very vigilent. I think it's great that you can probably deal with this by making sure blankets and clothing never sit around or get left on the floor. I'd get more litter boxes and experiment with better litters and maybe even buy a better brand of catfood if you aren't using something high end. It wouldn't hurt to use the science diet that was developed for cats prone to urinary tract infections. Then you'll be covering all the basics.
Don't forget that if the cat can smell any urine from previous times he's peed, then they will keep doing it in that spot. So you'll want to bleach the floors like crazy in those places. Just picking up the clothes may keep you from smelling it, but they have better noses than we do.
Goodluck.
Suzi