Lots of different reasons for everyone. For me, I left 10/31/2008. Remember, the markets were crashing so I was getting out while my company stock price was still high. It had dropped just under 2% in its 3rd quarter evaluation a month earlier, when I gave my notice. It would drop more in the 4th quarter evaluation before eventually rebounding (and I surely didn't want to keep working and waiting for that to happen). The company actually took frozen ESOP shares away from their employees at that time instead of giving them additional ones like they usually did at the end of every year.
The big bond fund I was planning to buy into with the proceeds of the company stock sale (an ESOP, so I had to sell it all, per plan rules) saw its price dropping a lot, so I was eager to buy into it at rock-bottom prices, something I have benefitting from for the last 15 years.
I didn't like the idea of commuting to work, even as little as 2 days a week, for another cold winter.
I was already on COBRA because I reduced my weekly hours worked from 20 to 12 back in mid-2007, making me ineligible for the group health plan. That COBRA plan was due to expire at the end of December (18 months), and I didn't want to have the twin burdens of working AND having to buy an expensive, individual, non-COBRA plan (pre-ACA, remember) at the same time.
My next annual bonus wasn't due until the following April, so there no sense sticking around for that.
One downside of leaving so late in the year was that the big ESOP payout threw the rest of my income, mostly wages, into the AMT territory, costing me an extra $5k. But I was able to use NUA (Net Unrealized Appreciation) on most of it which saved me a bundle in taxes.