Yeah, been there and... ummm, I'd guess you are really depressed. I know I was when that happened to me. I did not perceive it as random, cause actually there were some associations coming from the outside world that made me cry very very often (music, objects, random uncontrolled memories). Plus I was very stressed at the time, very very stressed. I think it's not random, but only you know your emotional state.
And friendly advice: *yells at you* do something before it gets worse! Seriously, ask for help, it's nothing to be ashamed of.