Why does the alarm go off randomly?

It could be a problem with low battery. Low battery can reset the clock and its previous alarms. Try installing a new battery and see if it solves your problem.

You probably hit the nap button by accident. My daughter had this issue and it took a while to figure out why the alarm goes off about 1.5 hours after she goes to sleep. If you hit the nap button you will see a n90-n80 and so on. This indicates how long until the buzzer goes off for a nap. Make sure you keep hitting the nap button until you get to off.