My Kennedy family did the same thing moving from Donegal--Ligonier--Lower Burrell--Washington twp--Freeport--Mon Valley within 1850-1870. The simple answer is that it was much simpler to move in those days, it was not easier mind you, just more simple (leases were informal, no utilites to transfer, school districts didnt exist, ect). If you lost work you had to move to find work IMMEDIATELY because there was no unemployment compensation.
Your ancestors probably only moved from Somerset--Stahlstown--Ligonier. Stahlstown was in Donegal twp, which was divided into Cook twp in the 1850's.