DIAGNOSIS This is almost certainly because the vacuum's stabilizer wheels on the back of the canister pod aren't deployed. They're not just for looks.
FIX Look at the back of the silver canister pod, near the bottom. You'll see two small, black, T-shaped plastic wheels. They fold flat against the body for storage. You need to swing them outwards so they click into a perpendicular position. When extended, these wheels act as a kickstand to counterbalance the weight when you're pulling on the hose.
IF THAT DIDN'T WORK If the wheels are out and it's still tippy, check that you're using the correct setup. The hose should be plugged into the port on the front of the canister, not the one on the handle. Also, make sure the floor nozzle is detached from the wand—having it dangling adds unbalanced weight.
HEADS UP Dragging the canister around by the hose, especially with the wheels still tucked in, can stress the hose connection point and eventually crack the plastic housing there.