I'd go with the other posts. Seems like uneven development - not sure what the cause would be. However, something off-topic, by the looks of those scans you seem to be underexposing your film. No detail in the shadows, muddy midtones, and practically no highlight/separation.
You should probably try and work on your exposure/processing technique. One thing I should point out -
If you are working with an in-camera meter, don't. Save yourself a lot of frustration and get a hand-held incident meter. In-camera meters (unless 3-deg spotmeters) are typically "averaging out" your scene. The reading is worthless.
I've been pulling my hair out for months until I realized this problem (had the same issue in my work).
My $0.02