I think this has little to do with sitemaps (other than in this case alerting googlebot to go and crawl the site, thus causing the coincidental drop in ranking).
BEFORE you move hosts you MUST setup your existing web pages and folders on your new server, read more here:
WHY choose the UK for web hosting?: Changing host - will it affect my Google search ranking or pr?
So many fail to do this! Fail here and you can lose months or even years worth of
SEO.
Do it right and your ranking will be safe, and if you are moving to a faster more reliable host then your ranking can and will improve.