shit, my bad! i just noticed you have a k5 600, not a k6... your bike does not have the slipper clutch, and therefore if you don't rev-match properly your rear tire will indeed lock up. i apologize dude...
when you are going for a downshift, you need to blip the throttle when you pull the clutch in to match the engine revs with the tire revs. if there is too much of a difference between the two, your tire will lock up and cause problems.
so the correct procedure should be:
1. apply front brake
2. pull in clutch
3. blip throttle while hitting your downshift
4. release clutch
you need to practice this so that you can do it quickly and smoothly, that will prevent any lock-up.