Seriously? Just to confim, turn off the wall stop valve before emptying next time.
If the toilet fills up again, then for sure you have a stoppage downstream, and pipes higher than the floor are storing up water to the level that the toilet refills to.
This may not seem like a blessing, but it may be. You get early warning to drain the septic or clean the drain pipes before the basement flooded. If the house has septic, consider getting that tank pumped as somewhat of an emergency.
There is one other possibility that comes to mind. If the basement toilet feeds into a septic basin with a pump, that could somehow figure into this. However the part of that does not seem to fit is the apparent storage of water above ground that serves to refill the toilet. But it is worth considering if that toilet pumps into a sealed septic pit.