1. Is it possible that the two breakers trip at much less than 15 amps ?
Possible, but in a house only a year old very unlikely.
2. Is there an easy way to test both circuits and determine if it is the breaker ?
Sure. 1800 watts is 120 volts x 15 amps. Get a portable electric heater or a hair drier. The high setting is usually 1500 watts. Plug that in and turn it on. If the circuit breaker doesn't trip, add 100-watt light bulbs until it does.
3. Can I just change both offices to 20 amp breakers ?
Probably not. 15-amp circuits are usually wired with 14-gauge wire. 20-amp circuits have to be wired with 12-gauge (heavier) wire.
4. Would this have something to do w/ the house being new and energy efficient ?
Are your "offices" intended as bedrooms? New houses are required by the electric codes to have something called "arc fault" protection. This is different from "ground fault protection" that's required near sinks. To be honest, I don't know just what arc fault protection is or protects against (arcs, I guess), but I have heard electricians complain that the arc fault breakers are a PITA. If you have them, they should be marked. Do the breakers that trip say anything about "arc fault"?
Many years ago my ex-wife and I moved into a new condo unit. About six months after we moved in, the breaker for the master bedroom began tripping at random. It took awhile, but I finally traced it to the outlet behind her bureau. "Production" electricians today don't use the screws on switches and receptacles. There are holes in the back and they just strip the end of the wire and "stab" it into the holes. There's a spring in there that's supposed to make contact.
If they strip the wire too small, or too short, there can be a bad contact. Or the spring can be too weak for reliable contact. I tossed the outlet, and used the screws to install the replacement. End of problem.