Mercury governs communication, technology, travel logistics, and signed agreements. During its retrograde period, these specific areas are traditionally considered prone to miscommunication and delays.
This is why astrologers commonly advise against signing major contracts during retrograde, and why “re” words apply: review, revise, reconnect, reschedule. It does not traditionally govern unrelated bad luck across all life areas — that broader usage has drifted from the original astrological scope.