strange kitchen experience

OK so i live w/ my boyfriend and we woke up two days ok. i went to the kitchen to make coffee. i set one mug on the coffee maker and the other mug on the counter next to it. i started the first cup and walked off to the bathroom. while i was in the bathroom i heard a loud thud. i thought maybe my boyfriend knocked something over in our room. walked back to the kitchen to make the second cup of coffee and the mug that was on the counter was tipped on its side. we experimented trying to debunk what happened (bc we were scared shitless). we tried putting water under the mug and seeing if maybe that's what made it fall over. bc normally when a cup/mug has water under it it will slide on the water. but not tip over right?? the mug didn't move no matter what we tried. CAN SOMEONE HELP EXPLAIN THIS WE CANT STOP THINKING ABOUT THE MUG