Without a lot more information, this is a total guess and should be viewed as nothing but a conversation amongst knife nuts.
Barring a failed design, it could be the heat treat. Did he get it hot enough? Did he temper it properly. If he's using an oven and a normal, well used steel....this isn't likely. If he's using the forge for the HT, it could be more likely. Did he get it into quench fast enough? There are so many things that can go wrong during a HT. However, very few truly ruin a blade from an established knife maker. If he didn't get the knife to quench quite fast enough, it wouldn't ruin the blade...at least typically it wouldn't. It just wouldn't be AS hard. I've seen bad HT's on knives...but they're not really common.
It could also be that it got too hot during grinding and ruined the temper. Certainly possible. I've smoked the tip on two knives. It's pretty obvious right away though. The steel turns colors and you see that immediately. If he was grinding with gloves on, this can happen easier than grinding without gloves.
It could also be a problem during forging. Too high of heat, for too long can decarb the steel. Need carbon to make steel.
It could be some type of contamination during forging.
It could be a bad piece of steel. It's happened before.
Anywho, my thoughts....although they might be worthless as I don't forge anything.