Who is controlling the 50-days & 100-days chart to follow this pattern? Are they Fund Managers from various investment firms?
No one "controls" these patterns - they are just reflective of market prices based on all trading activity. The analyst was making a prediction, not an assertion. They could likely be wrong.
Is this Moving Average pattern trustable so the S&P can't crash below the 100-days average?
No. It could go up a lot, go up a little, stay the same, go down but not past the 100-day average, or it could indeed go down and crash through the 100-day average. There's nothing that would prevent it from going through the 100-day average.
Are these drops expected like a few times every year or it is trigged by some major events?
It depends on whether you believe that technical analysis is deterministic or not (I do not). It might tend to happen every few years, but whether that's just coincidence or not is hard to know, since there are thousands of factors that affect the stock market as a whole.