Possible clue... I'm also hitting some key sequences that:
1. highlight all the text (I assume CTRL-A)
2. bump up the font size (I assume CTRL-+)
this is a little odd, you are the one pressing the keys right? so you must know what you did. unless either there is something very odd in your install or there is a plugin which is doing it then I dont think this is a roundcube issue. it is possible that you are either pressing the backspace key by accident or pressing the reload key (which ever that is for your browser)?
if i were you i'd go ask the admin, may be other people have had similar problems and he already knows about it or may be ther is a plugin which is doing something you dont know about.
Another clue -- I just found the 3 messages in the draft folders of SquirrelMail, the email app we can also use.
sounds right, draft messages are automatially saved periodically.