Alignment "jumping"
 

car was driving fine, I noticed on the way to work the steering wheel was off center, about 10 degrees. by the time I got home (about 50 mile round trip), it was about 20 degrees off. took it into the shop for an alignment and on the way it was hard to hold it steady.
they were able to align it and it drives ok for now, nothing obvious wrong with steering or suspension component, broken or bent tie rods etc... but I am wondering what could have caused it to get out of alignment so fast. the roads around here are rough, lots of bumps, bad patchwork and railroad tracks. but they always have been and it never got this bad out of alignment before.
the only thing I can think of that would not show to an basic inspection is the steering rack. if it is worn or slipping could that cause this issue?

They may have forgotten to tighten one of the tie rod adjustments.

I hope that is what it is. it is good now that they re-aligned it. but I have only driven it 10 miles or so. big snow coming in tonight, so it may be a while before I get some miles on it.