I believe all mammals can contract rabies. Though deer are considered herbivores, they eat meat on occasion. Birds, insects, occasional carrion.
If a deer is bitten by a rabid coyote or skunk, it can get rabies.
All mammals can contract and spread rabies, but it’s rare in deer due to their diet as you pointed out. I wonder if rather than contracting it from consuming meat if the deer had maybe run across a rabid raccoon? Maybe it was attacked by something big enough to bite the deer, but that the deer could otherwise defend itself against as long as the bite wasn’t in a critical place? Maybe it could have simply been a “nip” by an animal that is usually not bothered by the presence of deer, or something?