I am often switching brom brush tool to line tool and back to brush. Using the shortcut "u" sometimes calls up the custom shape tool instead of the line tool. This annoying and slows down the drawing process.
The problem you are describing is actually how the feature works, but there is a way to adjust it to your liking. You can go to Edit > Toolbars and define what the shortcut U toggles. By default, U calls up the Shape tool, which means if you hit SHIFT by accident at the same time, you'll toggle THROUGH the various other shape tools. If you don't want this, simply go to the dialog and remove the other tools from the group for U.