Your skill set, expertise, diligence, and social fit to the workplace is the product you offer; the product an employer buys.  Like any product, it is subject to the laws of supply and demand.

Beyond that, acquisitions, out sourcing, and economic downturns can trigger a layoff.

So always keep your product in demand and know where the demand is the most for it.  Every 5 years, run your own little market analysis for the product you offer to see if you should exercise your side of that "at-will" agreement you signed.