madhouse 74, as Bruce said, what you have reported is kickback, which means the engine is sparking before the piston reaches the top of its stroke (as it should) but either the spark is occurring earlier than intended, or the engine has less rotational inertia than intended, so the piston never reaches the top of the stroke - it goes back the way it came instead.
As Bruce has said, the most likely cause of this is that the engine has less inertia than it should have, because the blade plate is not rotating with the crankshaft. If the blade plate is present and firmly attached so it can't slip, your ignition timing has somehow become over-advanced, which can only happen with modern mowers if something is broken. For example, if it has a Briggs & Stratton engine and the flywheel key has sheared, this could well be the outcome.
Please let us know what engine this is and whether the blade plate is loose. If it is not, and you have a Briggs engine with a sheared flywheel key, it is fairly easily and quite cheaply fixed by fitting a new key.