what makes the oil pressure go down?

Oil pressure rises and falls with rpm. An oil pump is rpm based so the faster the engine spins the more pressure it makes usually limited to a certain point. What makes oil pressure lower (new engine as compared to a engine with 200k miles for example) is bearing clearances have opened up from use and causing oil pressure to be lower. If it is much lower there can be a few underlying issues.

Oil pressure also changes with the temperature of the oil. On a cold wintry morning the oil pressure will higher especially at idle until the oil warms up.