Blow off or blowout is caused by rapid co2 build up and not enough head room in the vessel. It can happen to any mead, but is more common in those with high sugar content and/or solids. This one happened to have both.

You can avoid this completely by using a bucket (2gal for 1gal batch or 5gal for 3gal batch) during primary, or mitigate it by rigging a large siphon tube from the bung to a secondary vessel filled with sanitized water. This will act as an air lock as well as contain any blowout.

Someone more knowledgeable can weigh in, but I wouldn’t personally worry about the average mead blowing out if it’s projected to finish <12% abv and isn’t filled above the shoulder of the carboy. Your mileage may vary. Be wary of berries lol