While a few of the potential reasons are mentioned above by other members, here is a more comprehensive list of what might be causing the problem.
1) Low tire pressure
2) The wheels are out of alignment
3) The power steering box or rack or power steering pump is bad. You can repair or replace..whatever needed
4) The fluid level in the power steering reservoir is low (atleast at 100K, you should replace the fluid..regardless if its full or not)
5) The power steering drive belt is damaged or broken.
6) The steering gear needs to be lubricated or repaired.
I cant think of anythng else....but if you go through this list...i am sure you will diagnose the problem..GOod luck.