Hi,
This is the sort of things that can sadly happen when dealing with multiple desktop environments on the same install. Lately there's been several posts here about weird bugs in MATE happening when installing/removing multiple DEs.
Way back, I had installed KDE and GNOME on a MATE machine. Removing it was painful and left stuff here and there amongst other bugs, leading me to a fresh install.
So, when you have the urge to try something else (and with DEs, WMs and tiling WMs, there are so many things to try in the Linux world...) you probably will not be satisfied with and ultimately will be back to MATE (this seems to happen a lot with this great DE 
), the best thing to do is to use a VM (the Quickemu project from Wimpy with hardware acceleration is very nice) or boot a live session so that you don't touch your install.