Enzymes are in cells. They're very organised and when the cell is rather brutally cut up I imagine they mix with things and they could result in things being broken down. If you put them in a cold isotonic buffer then they have less kinetic energy which means less enzyme substrate complexes being formed and thus the activity of the enzyme being reduced. If your exam board doesn't pick up on it being cool, then I assume you don't need to know this.
Buffer doesn't affect kinetic energy really. All it does it keeps the homogenate at a cellular pH, approx. 7.4 (dependent on cell type). This will stop lysis and stuff.