When moving tasks between columns in a work board, sometimes the task priority is set to high regardless of what it's currently set to. (I'm not sure if it's always the value high, or if it's using the value of another task on the destination column. I've not kept close enough track to be sure.)
This doesn't always happen. When it starts happening, it usually happens consistently for a while. It appears to be correlated with work board column moves being very slow to complete: I'll move the task to a different work board column and it will be shown greyed out for quite some time (30 seconds or more), and then will appear in normal shading with a changed priority. When this happens, the priority is changed inside the task as well (not just on the work board display), email is sent to subscribers about the changed priority, etc.