It's related to the StumbleUpon toolbar for Chrome. Apparently, if the search_engines.pm doesn't specify a query string for a search engine, and yet there is a query string, AWStats uses the first part of it, which is &device=chromebar in SU referrals.
Changing, in search_engines.pm:
'stumbleupon','',
to:
'stumbleupon','qwerty=',
(or any random, non-existing string instead of qwerty) fixes the problem, but you may want a cleaner solution for it.
More details here: http://winterdrake.com/awstats-tip-removing-chromebar-from-search-keyphrases/