Answer:
Because of inertia
Explanation:
As stated in Newton first law "An object at rest will stay at rest, an object in motion will stay in motion unless it is acted upon by a force."
For an example:
If you have a drink without a lid in a car and a car suddently the car stop, u will automatically try to catch the drink preventing it from spilling. This is because that the drink that was in motion with the car want to stay in motion, but when the car stop, the drink still wanna be in motion, that's why it will be flying off and spill if u don't catch it.
Back to the question:
if a bicyccle is in motion, it will wants to be in motion untill a force act upon it, let's say a bump or fricition.
hope this help. give me the brainliest if u can.