I'm really not an expert in this area, but from my own experience when i jumped on a bike for the first time:
- it could happen if you're releasing the clutch really fast
- jumping on the throttle hard when you're still releasing the clutch
I hate shoddy gear changes in cars, so once I picked up my bike I spent some time really practising and getting to know the bike so my changes were smooth.
If you think it's down to technique, I'd suggest some conscious practising - experimenting with how far you pull in the clutch and when you get back on the throttle etc. sure you'll soon find a happy medium for you and your bike.