That's a nasty disaster, frustrating, too.
But here's what I've seen that might explain why it happened that way. On a wounded hunter search I found wolf tracks, followed them and ran into a WOUNDED wolf. I went forward at him and he disappeared into some woods. The next thing I see is battle sounds behind me where my dogs were tethered. The wolf had gone behind me! One dead dog later I finally finished him off.
Another time, I encountered a Njerp. I went stealth and followed his tracks in thick woods. The next thing I hear is a death rattle behind me. He'd circled a woody area and was coming behind me for an ambush. Luckily, my Companion was a bit slow and lagged behind. HE encountered and killed the Njerp before he could do me in. At the time, I thought it was pretty cool that the AI would do this.
I guess your best chance in your situation would be a broadhead arrow at his legs, hoping for a cripple damage result. Sorry about your stunning result.