Conservation of angular momentum explains why liquid spins faster the closer it gets to the drain, but it doesn't explain why the liquid starts spinning in the first place. In principle, you could have a reservoir drain liquid without spinning whatsoever. So why then does it always spin in the real world? Well that's because the real world isn't so perfectly symmetric, like in the ideal case. This means that there's a good chance some liquid will fall down the drain at an angle, as opposed to straight down. The liquid that falls at an angle will push on the liquid next to it, which in turn pushes on the next bit of liquid and so on, all around in a circle. This effect reinforces itself, and so even a tiny amount of asymmetry will determine the net spin. What’s important is to realize is that liquids are sticky, especially water, and so there’s not just pushing going on, but pulling too. Not only that, but a spin that starts further down the drain can propagate upward and affect the water that has yet to fall in. This happens for the same reason, because water is sticky, and so it’s best to visualize it not as a huge number of individual particles, but more as a stretchy interconnected substance. This is exaggeratory, but it explains how spins begin.