Check the foundation?

I have a front door like that as well, and I know it's because my foundation on that corner wasn't deep enough or sound enough for the soil type, and is sinking. I braced the framing up a couple of years ago to square it, and it took about a year to go back to the same problem. Which means some excavation and concrete work. Maybe this summer I'll get to it.

From previous experience I expect it to take a few weekends with a shovel, and about $500 in materials, but it should work out fine. It's a fairly common problem in older houses in my area.