Pyruvate can be converted to lactate instead of going to the preparatory reaction. Why does this occur?

Pyruvate can be converted to lactate instead of going to the preparatory reaction. This does occur because Oxygen is not available.

When there is insufficient oxygen in the muscle cells for any further oxidation of pyruvate and NADH formed in glycolysis, NAD + is regenerated from NADH by reducing pyruvate to lactate. The enzyme lactate dehydrogenase transforms lactate to pyruvate