This might be because there were 2 or 3 (depending on if someone bought up tank slots) tanks already in play, or that someone bought a tank right before you did and was about to spawn in as one. I'm not sure who gets to be tank if the max is going to be hit though. For example, if only 2 tanks are available, and two people buy a tank and a scripted one is about to spawn in, I'm not sure who has precedence. I know the scripted one may, but I'm not sure if one of the players gets to spawn in based on who bought it first, or who spawns in first. IMO I think it's based on who spawns in first. I've bought a tank when 1 was already up, but when I was about to spawn in, another tank appeared and I spawned in as a jockey instead (you morph into tank AFTER you spawn).