I've hunted in Canada a lot. We cross the border by car and spend the usual amount of time in the check station, as we cross over. Over the years I have seen many guys get turned back. It's not just DWI, they will turn you back for all kinds of things. A group of 4 guys that were in camp only had 3 get across. The 4th guy said they found a decades old charge against him for having sex as a minor, which was illegal at the time. He said it wasn't even true and that the case was thrown out and the records sealed. But somehow, 40yrs later, a partial record emerged.
This guy was well connected and used all of his resources to get the matter cleared up. It took 4 days for the situation to be resolved and he made it to the camp on his last day.
Occasionally, I travel with 1 or 2 new guys that haven't crossed before. I tell them that if there is any reason that you may be denied entry that we're all screwed because I just can't leave a guy at the border. If there is any question about a new guy, I won't travel with them.
Lastly, I asked one of the more talkative Canadian Border Guards; why all these issues with stopping people from crossing because of decades old minor issues. He said it really got ramped up with the Canadian Border Stations when the US Border Stations imposed ridiculously stringent restrictions on Canadians who were crossing into the US. This apparently all started some time after the 911 attack.