A paralyzed rider does not fall off their mount
The only situations given for forceful dismounts are:
If an effect moves your mount against its will while you're on it, you must succeed on a DC 10 Dexterity saving throw or fall off the mount, landing prone in a space within 5 feet of it. If you're knocked prone while mounted, you must make the same saving throw.
If your mount is knocked prone, you can use your reaction to dismount it as it falls and land on your feet. Otherwise, you are dismounted and fall prone in a space within 5 feet it.
The only thing you can do directly to the rider to forcefully dismount them is to prone them and hope that they fail the DC 10 Dexterity saving throw.
However, it's worth noting that a paralyzed, petrified, stunned, or unconscious rider will automatically fail that Dexterity saving throw if you prone them or if you move the mount against it's will.