Let me run it down as I understand it.

Your Grandpa and you take care of this guys place.
You have permission to hunt.
The landowner also gave someone else permission.
You knew someone was comming but since he didn't show you thought he should youwent hunting.

My guess is:
The other guy probably thought he had exclusive rights. Pluss on 20 acres its pretty small.He was headed out for an evening hunt expecting noone to be tromping around on it scaring anything on it off. If he got an threw a fit with the landowner then he sounds pretty petty. If I read right its the landowners boss. If so he's probably got no spine when it comes to his boss even outside of work and on his own property. So this guy probably let him have it just as if they were at work (I expect he's realy trying to kiss up to his boss) So he passes on the favor and cuts into Grandpa.

If this is the case I'd have a hard time going back. I don't take being talked to like that to well. 20 years or not I would have problems if I thought it could happen again. Now if I talked to the landowner and knew it would not happen again then I probably would go back. It just all depends how much you trust the guy. I'd also aprecieate knowing when and if anyone else would be on the property so no confusion happens again. If it takes giving my phone number out to others with permission to hunt. Also I would make it known to the landowner to let everyone known that I may be there etc.

I'm sure there is much more to the story we arent getting. And don't take offence to people commenting on your typing. Most people want to help butyour lack of punctuation makes it very confusing.It may have made sense to you when typing but it is hard to reread for anyone else. Just take your time and try to get eachpunctuation in there as best you can.