Company A got their funding pulled and shut down. Company B, where I was actually at for about a year and a half, switched owners and shutdown my entire office. Company C merged into it's main competitor and effectively fired most of us.
I will admit I was at one fantastic job and after around 3 years I probably could of stayed indefinitely. But back then I didn't recognize the value of a solid job. If you land somewhere and you're well liked by people, and able to do quality work, you really should just stay there instead of chasing slightly more money.