Hello there!
When you break it down, a motor is a coil. The equation for a coil is V = L(dI/dt). What this means is that when power is first applied to the coil(we'll say motor for now), a large amount of current is required to start the motor shaft turning. This starting current is also sometimes called the kickstart current, and is several times higher than the normal running current.
Whatever motor-driver or H-Bridge you are using must be able to handle both of these current ratings. Do you know the kickstart and normal running currents for your motor? If not, try to find a datasheet online.