Hello Emad
TCP is designed to function this way. Remember that TCP is a reliable protocol which means that each segment that is sent is acknowledged, meaning that the sender is informed when data has successfully reached the receiver. If the sender is informed that for some reason a sent segment did not reach its destination (i.e. segments are dropped), it will respond by reducing the window size to one segment, as described in the lesson.
So TCP by design will respond to dropped segments by decreasing the window size.
I hope this has been helpful!
Laz