The cause is... randomness.

People make the mistake of thinking that 'random' means that both options are not just equally likely, but will actually happen equally. This is just not true.

Random patterns are almost always categorized by 'clumps' (the same result happening over and over). It's only when everything is averaged out that you see that the two options did in fact occur roughly half the time each.

Just because when you flip a coin you are equally likely to get either heads or tails, doesn't mean that you won't get 20 tails in a row. Unfortunately people do assume that that's what it means.