The 8 pointer I got last week did the same thing...

When he hit the ground he moaned for 10 seconds, but unknowingly, remained alive for 20 minutes until I got to him.
The reason it took me so long to get to the buck is that he had not moved since the 1st few minutes after shooting him, so I thought he was dead.
I ended up taking my time, packing up my climber, only to realize the buck was still alive when I walked up to him.
Of course when I was 20 feet from the buck he started kicking his front feet, which scared the Ba-Jesus out of me.
Well now I had to pull out my old Shrade knife (as I left my gun by my stand 75 yards away) and proceed to punch out his air bags, not very fun, but effective.
30 seconds later he died and I finished dressing him out @ 141lbs.

I hit high, in back of the shoulder between his spine and top of the lungs.
I like to call this area the "free space", as deer could live quite awhile without a lethal organ shot.
I believe the trauma from my 50 caliber muzzle loader must have paralyzed his hind end, as there was no hole in his spine, so in my case I was lucky he dropped in his tracks.