I just got new struts on my 2000 Honda accord. Next day I heard a pop sound and found out my axle and mounts are broken. Is it possible it was cause of not properly putting on the struts? Or something that would happen?

What mounts broke? I'm not sure the connection between replacing the struts and an axle breaking, unless the strut was not energized first. What part of the axle broke?

On Hondas, the through-bolt on the lower control arm should be properly torqued, when the car is on the ground. This is often not done but I've never heard of consequences like you experienced.