It was definatly enough chemicals in the tank. However, I re-used the chemicals from a previous development.. But I somewhat feel like this could be camera-related. Am I totally wrong thinking like that?
Don't forget that the developer has to wet the whole of the dry film, the reels and the tank, so that a small, but not insignificant, is retained when you pour it out.
So, when used a second time, the volume might be reduced enough to leave poor development on the top edge.
Otherwise, residual photoflow, as suggested by others, would be a possibility. I can't think of any obvious camera issue which might cause anything similar.
OTOH, on a second look, there does seem to also be some marking near the other edge of the frame, which might accord wth film damage such as dampness, as suggested by MattKing ?