Why did my halva crumble?
Brands of tahini can vary; you might try adding a bit of additional oil (sesame oil would be a natural choice).

The ambient humidity where you made this may also have contributed.  If you're working in a dry area or kitchen, you may need to add another ounce or so of water to compensate.

Lastly, you might also (gently) try heating the tahini and proofing your work bowl with a bit of hot water to keep everything as warm and pliable as possible while it mixes.