You should contact the Bing team to follow up with this:
https://support.discoverbing.com/eform.aspx?productKey=bingw...
I used to run the Bing Webmaster program, and I know they do support crawl-delay. It could be someone masquerading as BingBot, or simply a bug. Ping me if no one gets back to you and I can follow up.
We see nearly a thousand parallel connections from their bots, nearly continuously.
For only 18% of search use, they seem to consume several times more resources than Google.
And for some stupid reason their bing bot and msn bot do not share data but crawl the same pages independently.
(and yes we use crawl-delay directive, they ignore it)