Okuridashi is quite often the result of an attack gone wrong: e.g. an attempted oshidashi or tsukidashi (the attacker tries to force his opponent backwards out of the ring) where the attacked rikishi (wrestler, lit. "strong man") evades the attack, and manages to get behind his opponent to push him out of the ring. "Okuri" means "pushing on the back", and "dashi" is "pushing out of the dohyo".
If the loser falls when pushed in this manner, it's okuritaoshi - "taoshi" being "making opponent fall onto his back or side" (except for this one instance, where he is likely to fall more or less on his face!).
All "okuri-"-kimarite are initiated from behind the loser - throws, pushes, pulls, slappings, and whatnot. Just being behind the loser does not make it an okuri-move though: there must be bodycontact.
Okuridashi cannot be said to be a technique more favoured by any one rikishi. Many an okuridashi is the result of an overzealous attacker being "outsmarted", or a quicker wrestler working his way around a somewhat slower - in fact, this may be the case with most "okuri"-techniques.
Of 571 bouts in the Makuuchi division (Haru and Natsu Basho, 2005), 18 (3%) were won by okuridashi.
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My sources are www.scgroup.com/sumo and www.sumo.or.jp/eng/index.html