A Bing Help thread has a solo report of an issue with Bing leaving new sitemaps in the "pending" status.
Sitemaps can be submitted to Bing via their Webmaster Tools interface. The issue is, if you recently submitted a sitemap file, it may just be sitting in "pending" status.
Will from Bing confirmed the issue saying:
Our product team is aware of this and is being addressed at this time. Currently, we have no ETA yet due to testing and developing.
It is hard to tell how far back the issue goes but there is an Bing Help.