My guess is that they ALL aren't loose, but a lot are. You may have a flat spot on your rim.
Give your wheel a spin and use your brake pad as a focus point to see if your rim has a flat spot which means your wheel is not longer perfectly round.
It is especially hard to feel this on a mountain bike, but on a road bike with a severe case, you can notice it on long rides on a good rode; whumpa bumpa whumpa bumpa.