Why did this roux fail?

I needed to make gumbo for someone with a gluten allergy, so I used glutinous rice flour in the roux instead of wheat flour. At first it looked ok:

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When I added the "trinity," though, this happened:

Basically, the roux split, and the rice flour formed disgusting blackened clumps. Here’s a close-up:

Yeah, I didn’t like looking at that either. I tried again with tapioca flour, and that did work, but I’m still wondering why that happened with the rice flour. Could it have been some kind of thermal shock when the veggies went in? Or maybe a reaction to some chemical in the onion?