You have probably not calibrated your throttle(s). If not, the reversers engage as you approach idle on the throttles. Check it out on the end of the runway from an exterior view. If you spawn on the runway with throttles back and the reversers are activated, you have to calibrate your throttles so they do not activate until you want them too. There are several informative youtube videos on how to use the EFB (“Ipad tablet”) in the cockpit to calibrate the throttles for TOGA, FLX, CLIMB, etc.