circuit breaker issue
The simple answer is that the breaker is tripping because two much current is flowing through it.

Here's a list of some possible causes for over-current:


Faulty wiring may be partly short-circuiting
Faulty appliance is drawing too much current
Too many devices on that circuit, in total drawing too much power


I would start by proving to yourself that there is definitely nothing else on that circuit other than the opener and the lights. Turn the breaker off and then go around and see if anything else is off. Outlets in the garage? Chest freezer? Sump pump? 

Second, see if you notice a pattern of when the breaker trips. If it trips when you flip the light switch, then maybe the switch is faulty or maybe the lights are the extra current.

If you're positive there is nothing on the circuit, then you're looking at a defect. Finding it may or may not be in your skill set, but you could start checking the voltage at the opener with a multimeter. Then start opening outlet covers to see if there are any lose wires. Check for signs of arcing (which may leave black sooty marks around the outlet terminals.

FYI, the time it takes to trip a breaker depends in part on how much over-current there is. It may take several seconds or even a minute if you're only just barely over the circuit breaker rating.