Thank you for your question on the kick back on the starter recoil.
Please make sure the underside of the mower deck is clean.
It sounds like your flywheel key may be partially sheared.
If you had the mower on its side to sharpen the blade you may have gotten oil in the cylinder.
First remove the spark plug wire and remove the spark plug.
Then pull the engine over to see if any oil spits out of the spark plug hole.
If it did not then I think during the operation of the mower something hit the blade hard enough to partially shear the flywheel key so it is out of time and kicking back.
In order to replace the flywheel key you will need to remove the top sheet metal and covers from the engine to be able to get to the flywheel.
You may want to take the mower in for service to have the flywheel key changed. In some cases you will need a special fly wheel puller to remove the flywheel.