Only pump it up to 1-2psi so that it holds it shape, but you can still move the tyre around on the rim. Push up on the part that's depressed into the rim. You'll want an even border at the rim/tyre interface. Then do the other side. Spin it to make sure it's even. Then gently install the pump-head and give it about 10-pump. Spin it again to make sure it's even. Then pump it up the rest of the way.

The 26" tyre and rims have a lot of tolerance, so it's possible to have a lot of play and when that play all stacks up on one side, you get the wobbles.