Smartphone bot crawl spike

I'm seeing what looks like a biweekly Smartphone bot crawl throttle every 14 days starting Dec. 19/20 that I can't account for. The site isn't being updated more than usual on this schedule (we are a news site so we publish 100s of pages daily), the sitemap isn't being updated on this schedule (we have a news sitemap updated daily), the site isn't being redesigned, we don't generate dynamic links, etc. Crawl requests, for the most part, don't seem out of the ordinary. Any ideas what could be causing this? TIA!