Zhou Wen could tell that the three-eyed antelope wasn't using much force. Since its bite could chew through the tenacious withered grass, his backpack would have been torn to pieces if it exerted a little strength. It wouldn't remain in one piece as it was now.
The antelope bit on the zipper in an attempt to unzip it. It most likely had its sights on the withered grass that Zhou Wen had dug up.
Zhou Wen took off his backpack and carried it as he continued walking up the mountain with his back facing the three-eyed antelope. After all, Mount Laojun's dimensional creatures weren't violent, so he wasn't afraid it would do something to him.
The antelope clearly wasn't giving up as it circled Zhou Wen and attempted to reach its head into his backpack.