Why some stocks change direction ?

This is a general question. Consider 2 stocks A & B both belonging to BSE A Group. On a particular day, both start rising until 10am. Then A continues to rise but B changes direction & starts falling. What is the general reason for this change in direction ?

As a clear example of what I have said above, many stocks reversed direction between 10.30 & 11.00 today on both NSE & BSE.

What was the strong reason for this ?

What should one do in such a case if they have opened positions before the reversal ? Should they helplessly allow the stocks to hit stop loss price ?