Why did my halva crumble?

I followed this recipe (Hebrew; my loose translation below) for homemade halva, which is a candy based on tahini (ground sesame seeds). As you can see in the picture in the original recipe, the end product is supposed to have a rather firm consistency. I also understand from the recipe that after the mixing stage, when the mixture is poured into the pan, the warm mixture is supposed to be rather liquid.

1. Place 500g raw tahini in a mixer bowl.
2. Heat 500g white sugar with 1 cup water up to 121°C.
3. Set mixer to medium speed, and slowly drizzle the hot sugar syrup into the bowl.
4. Add 1 tbsp vanilla extract, and mix for 30 seconds until combined.
5. Add 1 cup peeled, roasted pistachios and mix until evenly dispersed.
6. Pour the mixture into a square pan, lined with a baking sheet sprayed with cooking spray. Cover with a second baking sheet sprayed with oil and let cool to room temperature.
7. Cut to small cubes and serve.

In my attempt to make this, the mixture became quite firm while still in the mixer, even before I finished drizzling all of the sugar syrup into it. I insisted on drizzling all of it, and by the time I was done (and combined the last ingredients) the mixture pretty much turned into halva crumbles. I tried pressing it into shape in a pan, but the crumbly mixture remained just as crumbly after it cooled down.

Can you figure out what I [could] have done wrong? Perhaps I have cooked the sugar syrup too long? I used a thermometer, and I took care not to touch the bottom of the pot. Or have I drizzled the syrup too slowly?

I'll appreciate any input.