Great question, and I look forward to seeing a Roman history expect address this.

I wonder about a possible parallel: a recent online article in Archaeology by David Powell discusses the work of Archaeologists David Anthony and Dorcas Brown, who suggest "that in the Indo-European tradition, eight warriors may have been the ideal complement of youthful war bands." The evidence presented in the article is intriguing and might, perhaps, point to a deeper cultural bedrock that influenced Roman military tradition.

Presumably you're referring to the contubernium, which was indeed the smallest organizational unit in the Roman army. However, it was not a tactical unit. The lowest tactical unit was the century of 80 men, led by a centurion; the decanus in charge of the contubernium had no direct authority in battle.

As to why eight men in a contubernium? Because that's how many you could fit in the tent. That's all the contubernium was. It was a group of eight guys who slept in the same tent when the army made camp, shared cooking responsibilities, etc. Same concept as the "messes" of the 18th-19th century - a small group of soldiers who eat together, share some basic tasks (you two pitch the tent and I'll start the fire), but has no tactical function.