Another possibility is some change in the environment that might seem trivial to you, but isn't to him. It could be scent (new detergent or air fresheners?), sound (construction nearby?), food, housemates (human or animal), almost anything ... It might not be an ongoing change either--if something happened to disturb him, he might remain on alert for a long time.
If I couldn't think of anything, I might have a vet exam and bloodwork done, in case there is a medical issue.
I have a 13 year old whose behaviour suddenly changed in the spring--she would suddenly jump up and switch locations, she spent more time in her crate, and so on. In her case, the problem seemed to be pain. She went to the clinic 4 times and told 4 different stories about what hurt. She's now on a cocktail of pain meds and supplements. That, plus the cooler weather, and maybe time (perhaps she had a strain or nerve impingement that's healing) seems to have helped.