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That's unusual, especially considering you weren't actually firing the flash at the time, from your description it sounds like you hadn't been firing the flash rapidly in the lead-up to failure.
It's odd that the batteries were hot, that indicates high current through the batteries, perhaps there was a short-circuit within the flash (either before or after failure).
A flash failure can be caused by a high trigger voltage, but you weren't actually triggering the flash at the time (I assume you've used the Yongnuo triggers before without issue)
I expect that repair may be uneconomical, but you can always ask
Sorry I can't be more help