Wheel Bearings coming loose

I installed a new front suspension in my 66 Polara and when I took it out for a road test, it had a vibration. I had new tires (for the car, but nos bias ply) and had it aligned. It was fine at first then the vibe started. Checked the tires and they checked out ok. I had previously checked the bearings and they were tight. Afte checking the tires I checked the bearings and got more than a full turn more when tightening them. I took it out for another test, and the vibe was there again after 5-6 K miles. I didn't look/check, but I know they are loose. What would cause this problem? I've replaced bearings before without incident so I'm kind of confused. Thanks in advance.

Scott