I wonder.... Is it target panic? An example of target panic would be when you shoot many arrows at targets, and it becomes increasingly harder to come to full draw the more arrows you shoot. Eventually, you become mentally unable to come to full draw, even though you are physically capable of doing so. That doesn't sound like what happened to you, unless you continue to experience the problem at targets other than the buffalo you shot at.
Could it be that you just blew the shot, without any deeper meaning to it? We all blow shots, and hope that when we do, it's not when we're drawing down on a prize animal, but there's no guarantees.
Could be target panic, but not if you're not continuing to experience the problem after coming home. Could be buck fever, but you've taken buffalo before, without experiencing that.
There are ways of dealing with those issues, but those issues are chronic problems, and dealing with them involves methods of breaking ingrained habits.
Dealing with an occasional blown shot is different; it involves an acceptance of the reality that those things will happen, and they won't have any effect on your next shot unless you let them.