Elk are very unpredicatable but from the info you gave it sounds like that bull sensed you in some way, probably heard you, and just wanted to know what you were. He probably was looking for a response of some kind. Thats when you just give a little cow call to calm his nerves or imitate him if you can. Sounds like if you would have given a smaller bugle he probably would've wanted to fight you and he'd be in your freezer. Elk are very smart though, if he can't see you don't call unless you are in sufficient cover. Reason why is even if he can't see you if there isn't enough cover to hide an elk he'll know you are a fraud.