This applies to new hamsters more than older ones, but still occasionally happens with Munchkin. Hamsters can be very jumpy around us heavy footed humans simply because we are so much larger than them! If you approach them from behind or above they can't see you so you may make them jump, sometimes quite significantly.
When I first had Munchkin he was playing on my bed. I moved my leg and he shot into the air falling off the bed :(. Luckily he was fine, but it is something that can happen often when owning hamsters.
To prevent your hamster being jumpy:
- Make sure you talk to it a lot so it gets used to your voice
- Never approach it from behind or above
- When picking it up ensure they see your hand first
- Try to avoid loud noises where possible