If you talk abt dramatic dips, then I can think abt 3 reasons:
2) a-la Dusnrogers: that is someone who plays at X-rating begins more than 100 games (preferably in tournaments, where all ranges of ratings exist), moves quickly in games against weaker opponents and slower against those who are stronger and wins game after game reaching (X + 200) and higher rating. Then very glad with his achievement he resigns all the games with worse positions and repeats it again
3) being kickedfrom the clan (not because being inactive - it would be first case) and resigning all clan games on purpose - just like "take my eye off, but take both from my neighbour". Of course it usually will result in one-event dip and return, but lately I read that there are few players who like to join clans and then leave them, certainly it is some kind of disease that can get name after its patient, that is "a-la ..." or " ... syndrome"