i was able to reproduce the issue (see also images with explanations)
http://imgur.com/a/B3L4E
what happens is basically this:
if you have a main melee weapon and you switch between your melee weapons via hotbar it is very likely that your main melee weapon will be removed from the "main" slot and replaced by a different "minor" melee weapon, thus leading to a accidental loss of your main melee weapon