Dave,
After listening to it multiple times, the bike sounds like its unloading to a lower gear, which obviously would spike the RPMs. What gear were you in when this happened? I had a 95 KX250 & the Gear Dogs were rounded, shifted fine & would only do this when it had a big load on it as your video shows.
Next Ride, pay close attention to what gear you are in, and when it happens back off to mid throttle & shift to the next gear until run out of gears, to see if it actually is jumping down a gear.
DoldGuy