Two factors come immediately to mind: Stimulus and Diet.
Visiting my family stresses me out, especially around the holidays. I have three young (and therefore high-energy) siblings, an extroverted older sister, and a mother whose demeanor is "a little on the abrasive side." At least two people are talking at any given moment, and the TV is often on, while I am straining desperately to focus on some important and elaborate personal task. And if someone touches me... that will push me over the edge almost every time.
If there is simply too much going on around you, remove yourself for awhile. Find a closet or a coffee shop, some controlled environment where you can sit alone and recharge.
I'd also recommend transitioning to a gluten-free casein (milk protein)-free diet.
In addition to Asperger's, I also have a digestive disorder and have become a vegan as a result. For me, consuming wheat or dairy results in a psychological/emotional crash within 24 hours.
My mood swings are not 100% under control yet, but identifying triggers (whether in your control or not) and avoiding them as often as possible is an important step to regaining stability.