Mature Russian Boar will kill sheep, goats and baby calves. It is possible that they kill fawn deer also. However, it isn't likely that they killed mature deer, possible I suppose, but not likely.
They are true omnivores and will eat anything, including each other under the right circumstances.
My guess is that the deer died nearby and was eaten by the hogs as basically a free meal. The wallow could be near a bedding area where natural die-off of deer could occur or the deer could have been injured.