Said middle manager would be higher up. There is no "middle" to manage at the gate. It's probably a bored jobsworth in the operations center that was keeping an eye on the @easyJet feed. Maybe he discussed it with a colleague or two and they stirred up a frenzy. Maybe someone had just been at sensitivity sensibility training and used his newfound knowledge to identify the tweet as threatening.

Many scenarios are possible, and we can't expect easyJet to air their dirty laundry, so we probably won't know exactly what. What is completely inconceivable is that there is a company-wide policy that came into effect (and was subsequently immediately withdrawn this morning) to hunt down and deny boarding to passengers that criticizes easyJet on Twitter.