I have a theory about this. Cats' time is very scheduled, and they carry tiny PDA's (Paw Digital Assistants), to which they download each day's schedule from the Mysterious Cat Planet via satellite link.
They'll be walking along, doing nothing, and suddenly their PDA alarm goes off and it's time to wash between the toes of their left hind foot, right this minute!
I used to believe in the cat motto: "When in doubt, wash," but now I believe in the Cat Planet. It's the only logical reason for why my cat goes ans just sits in the shower stall for a half hour at a time...it's her secret office.
Ok, seriously, washing is a very non-threatening activity, and a washing cat is not going to attack you. I have a condo full of cats at the shelter that don't relate to people at all (feral mom, brought in at about 3 months old) but who have excellent cat communication skills, and they'll often wash to indicate that they're not a threat to the one who's trying to come in from the porch, or that they're not interested in having a spat (these guys are my favourite shelter cats, I learn a lot from them).
Also, sometimes your fur just gets messed up and you have to fix it, like smoothing your hair after you take off a hat; it just doesn't feel quite right until you arrange it properly.