This looks like one of those questions to which the answer can only really be a guess, but I believe frogs have been known to be prey items for hedgehogs. I have also read that hoglets can be a bit more adventurous in what they eat than older hogs. I can only assume that the hoglet thought it might be an interesting thing to eat, but then realised that maybe it wasn’t (maybe toads are more distasteful than frogs). The taking back to the feeding station, may have been a conditioning thing, from having been accustomed to eating in a certain place – being recently released hogs from a long time in captivity – Just an idea, could be completely wrong!
Very sad that the poor thing had been hit by some sort of garden machinery. If only it could become a routine thing to check for creatures before strimming, etc., lots of them could be saved from unnecessary death.