Mega knight crush terminating Ram Rider's charge
Any knockback or stun interrupts the charge (pushing the unit alone does not). This is normal behavior and also stops the charge of other units, such as the (Dark) Prince and Bandit (if well timed). They will try to restart the charge, if there's still enough room for it or just attack the next target in range.
Using a Mega Knight to counter these chargers is usually a good idea, but there are cheaper cards with the same effect (e.g. Zap, which can be very effective when paired with a damage dealer).