I know that it must be disappointing to hear, but before anything is taken too seriously it's always a good idea to inspect rational reasons as to why the mirrors fall or get "knocked over". Is their placement what you'd consider unsteady? If they are hanging, are the nails securely placed? These are things to really inspect before going straight to spiritual contemplation.
No matter how hard people try to take care of their mirrors, glass does get fracture points in it over time and they will suddenly "snap" due to things like gravity and such. The jolt of the break can make them fall on their own, especially if they're already used or old. I'd say this might be the cause and is worth looking into.
If you truly believe it could be spiritual in nature, you might have a spirit in the house that used to live in it. How old is the house that you moved into? Are you or your friends very disrespectful to the property? Are you or your friends very negative people? Negativity of the living can affect the atmosphere of an area and even disrupt spirits. This might also be something to consider.
If you think it is a spirit I suggest doing any sort of general purification for the house and getting rid of the mirrors, buy new ones, and see what happens from there.