The only real possibles are:
Wheel nuts aren't tight enough, causing a little play.
Cones aren't tight enough - these can be fiddly to get right if you don't have thin cone spanners. In fact, they’re fiddly to get right even if you do have thin cone spanners. They need to be tight, but not too tight.
Rear triangle is loose - this can happen, but in my case was also accompanied with lots of squeaking.
Tyres need more air - I've been on holiday for two weeks and rode my bike yesterday without checking up on the tyres. The bike was squirming all over the place. Checked the tyres on my return and the rear was down to 40PSI and the front about 35.