Margaret, on an errand for her mother, finds herself at the Thornton's when a group of maddened strikers rushes the house. In the middle of an episode during which even Mrs. Thornton seems fearful, Margaret keeps her head. This puts her in complete contrast with Fanny, who is screaming and crying, making the servants even more panicky than they already are. Maintaining her dignity is very important to Margaret, and she is relieved that, when tested, she keeps it together.
When Margaret and Mr. Thornton look out the window at the mob, Thornton- understandably- sees a seething mass which is intent on doing violence. Margaret however, who has met some of the millworkers and picks out at least one in the crowd whom she knows, sees them more as individuals.
What Margaret doesn't realize until too late that, although a mob is made up of individuals, it does not behave the same way an individual would. A mob, emboldened by the anonymity of being in a large crowd, their rage feeding off of that of those surrounding them, will behave in ways which a single person on his own never would. One of the striking workers would never dream of physically attacking Mr. Thornton on the street, but as part of an enraged mob, many of them actually proceed to attempt to do so.
As mentioned, this incident causes Mr. Thornton to acknowledge his feelings for Margaret. For Margaret, however, waking up to hear Fanny and her maid salaciously discussing the incident, her first reaction is extreme embarrassment. She has always prided herself on being dignified and ladylike in her behaviour and now, by inserting herself between Mr. Thornton and the mob, she has made herself an object of gossip and speculation. At this point, her feelings for Thornton- whatever they may be- don't feature in her thoughts; she just wants to remove herself from the situation as quickly as possible.