Dr. Richards is using the expression “having a cow” to mean “becoming angry, excited, agitated” in in unnecessary sort of way. This is more of a North American expression. Good question, especially if you are not familiar with the expression - sounds kind of weird.
So, if we let ourselves get worked up over nothing, we’ll be getting overly angry, or excited, or agitated, or worried, or depressed over nothing (over-reaction). We’ll be “having a cow” over what could be a no-big-deal situation.
I think sometimes it’s good in therapy when we use expressions/terms that carry a sort of feeling or impact, or let us also see a visualization. Like when he says “don’t make mountains out of molehills”. If the expression helps us to remember to calm down, put things back into perspective - then it’s useful.
Hope that helps answer your question.