For what it's worth...I've had the experience of my bike jumping out of gear as well, both during acceleration as well as deceleration.
Popping out of gear during acceleration is more than likely to be a bike in need of the infamous "upgrade kit", it usually starts occurring in second gear under heavy torque load. The upgrade kit gears are "back cut" to help ensure tight enmeshment while engaged.
Popping out of gear during deceleration is most likely a weak or broken detent spring. This will happen in any gear during deceleration. You can keep it in gear if you maintain foot pressure on the shifter after making the shift. Never lifting your foot off the shifter keeps the required pressure on the fork to keep it in gear. It can be replaced without removing the engine...just the radiator and front cover.
I'm just sayin...