It's been my experience lately that there's two main circumstances in which developers can inccurr a significant amount of unhealthy work-related stress:
Nothing to do. This doesn't sound stressful, but when you go to work and don't have anything to do it is extremely unfullfilling. Not to mention the temptation to pull up inappropriate material from the web...
Working for other people's mistakes. When working on a team project, if you're having to work on code that is not yours, not your design, or dependant on others, this can be an extremely stressful environment. If you find that your project/tasks are simply garbage because of other people's inability to write clean code, it is sometimes difficult to show others the fault is not your own.
At least those have been the two main culprits of my work-related stress as of late, what's yours?