The idea is that market participants who use similar risk management techniques may respond to a perceived increase in the riskiness of their positions by paring back the size of those positions and perhaps by paring back positions in other markets as well.
Such a response, while rational from the perspective of individual market participants, may have the collective and unintended consequence of reducing market liquidity at the time when it is most needed. I recognize the relevance of the questions and realize that herding might theoretically occur in a number of markets.