You can also get "demotion bonuses."
The "Promotion" and "Demotion" events on the player's profile are not necessarily bonus/penalty events so much as the triggers that often (but not always) cause an unscheduled development update. Because a promotion constitutes "good experience" the resulting development update is usually positive, but not necessarily; likewise a player usually suffers from demotion, but if he was otherwise doing well (for example, a successful cup of coffee while replacing an injured player - where there isn't supposed to be a penalty for returning him to the minors) he can actually improve on an update triggered by a demotion.
You should note, however, that if your player recently experienced a "regular" development update (i.e. "Spring training" or "regular season") the chances are much lower that a promotion/demotion triggered update. I cannot prove it, but it seems to me that if there is no such update then the promotion/demotion effect is lost forever and does not factor into the next update. So ... whenever possible, if I have to demote I try to do it immediately after a scheduled update.