Probably there are some other mechanisms involved, but one regulation is the depletion of free NAD+. If you don’t have any oxigen to finally receive the electrons made on glicolisis - Krebs cycle, all NAD+ gets converted to NADH, and the reactions that requiere free NAD+ just stop. To avoid all these things, you can convert piruvate to lactic acid (lactic fermentation). This reaction uses one NADH, and converts it to NAD+ again, so that the glicolisis - lactic fermentation can continue. This reaction takes place in the cell cytoplasm, not on the mitochondria, separating in from other processes.