in EB (and in vanilla RTW) never happened for me for now, but in a Franks campaign with BI I had a lot of captain promotion, even the head of faction was on these former captains I think it depens on how many living and adult family member you have. In that campaign most of my familiy members died in some really bloody battles against the western romans and I was reduced to 3/4 members, forcing me to send captains in battles instead of generals: I had 4/5 promotions and after that (with a more or less decent number of generals) the promotions stopped.