The following question was posted on the expertlaw.com forum back in December 2011:
Can You Get Fired for Doing Something Outside of Work
The first response pretty much sums everything up nicely:
You can be fired for bringing flowers and a box of chocolates to your manager and telling them that they're the best manager in the world.
In other words, you can get fired for a good reason, a bad reason, or no reason at all.
If you don't think so, ask Olivia Sprauer.