Straining too hard on the turlet.
Really though, I try to enjoy my dumps. I'm guessing I just go too much because I eat a lot. I remember the subject came up and people were saying they take a dump every two days or some crazy shit.
Check the wiki; there are a number of causes listed, and some suggestions for relief. Many of them are pretty simple.
Also, you may need on of these:
http://www.toilet-related-ailments.com/order.html
spoiler (click to show/hide)
I keed. I keed. I live in Asia but I can't use these fuckers without substantial pants acrobatics.