If they were laid off repeatedly (with the company still surviving) it would make me question why their name always seemed to make it onto the layoff list.
If they changed jobs repeatedly every 1-2 years because they were bored then I'd be leery because I wouldn't be able to trust that I could give them responsibility or factor them into long range plans without them getting bored and leaving.
As a manager I would consider two years to be the minimum commitment for full time employment. If you (the candidate, in general) leave before then without extenuating circumstances I would probably skip over your resume the next time it landed on my desk down the road regardless of how much of a rock star you were.
Searching for candidates is an expensive, time-consuming process, I don't want to do it when I wasn't expecting to. Especially to fill a position that I recently filled.