If you've never had a real back injury, you have no idea... Speaking as a guy who has had back problems since an injury at the age of 16 (I am 53 now), I can tell you, back issues never go away and they sometimes pop up randomly from seemingly mundane things. As an example, I have been dropped to my knees in pain from simply bending over to pet the dog. So if Tiger has back issues, they aren't going away. Ever. He will continue to fight this for the rest of his life, let alone the rest of his professional golf career. So no, it is not odd that he would have back spasms or back pain similar to what he had last year. In fact, it will likely become far more common as he ages.
I would like to confirm the above. My injury happened at 16, I'm now almost 66. I can go for a couple of years w/o any real pain, then one day..BLAM! I could hardly stand up, much less hit a golf ball, and no matter what I did, nothing would cure it, maybe help some, but years ago I accepted the fact that I will always have some pain.
I just hope that Tiger's issue aren't as bad as mine, and his can be relieved somewhat so he can continue to play.