Bootvis accurately describes the math - Skew plus Kurtosis. What's interesting is that many of the efficient market theorists (example: Eugene Fama) observed this phenomenon too.
I consider two intuitive reasons for this:
1) Behavioral - According to prospect theory, the mental benefit of gaining a dollar is lower than the fear of losing a dollar. This means we're more likely to panic to avoid losing a dollar that irrationally join a gold rush. This is just a matter of degree though - the high Kurtosis means we're more likely to do either than a Gaussian distro suggests.
2) Leverage on Equity Bets - If you're levered up on equity 10 to 1, and your position drops 10%, you have to sell everything. The reverse doesn't exist. Many banks are leveraged even more than this. This means when bad things happen, there's system pressure which doesn't exist when good things happen. Or at least not as often - there are still short squeezes, but they seem small in comparison.