I don't know why that happens, but as a workaround you could use the Crop Tool to increase the canvas size.
how would yo do that?
Draw out the crop tool and then pull out one or more sides and commit the crop.
To expand the canvas on all sides press the alt (option) key and pull on one of the corner handles.
The downside is it's not as easy to expand by a certain amount.
right side of canvas expanded:
all sides expanded:
thanks lol. Wow I never new you could do that lol, so simple.