This is a Mac (and how it interacts with GMail) issue not a BlackBerry issue. People have reported it previously. I don't recall anyone posting here that they had found a fix. You might try some Mac forums.
I was having this problem as well. I went into my Preferences, then Account, and then Mailbox Behaviors, and then I unchecked the box that said "Store draft messages on the server". I don't seem to have this problem anymore.