Glad to hear you didn't come off any worse! Same happened to me a couple of years back from the ferry after a trip abroad. To be fair, my chain was near the end of it's life and I knew that on the way home. I kept having to tighten it up! Luckily it just came off without any damage to me.

My first thought was that your chain could have been slack (like mine was), but from what you have said you keep an eye on it and it's fairly new. But it could well have been that.

Like you said, the 3mm aluminium bracket could have fractured. I also have fitted a chain guide under the swingarm, but my bracket is made from 3mm thick stainless steel (fitted after my chain came off....).

Just out of interest, did you buy flat aluminium then bend it yourself or did you buy angled bracket?

The reason I ask it that I've had a 3mm thick aluminium bracket on my DRZ fail and fracture. I used it to hold a tool tube a while back before I made my own from stainless. The alloy, I suspect, was not great quality and it was bent along the grain rather than 90 degrees to it. The bracket failed at this point and I could see the fracture progression along the grain.

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