Judo contains a number of rituals inherited from Japanese culture, one example being the codification of bowing to your opponent before and after competing.1
However bowing doesn't carry the same obvious connotations of respect to many non-Japanese judoka, and so there is a tendency to shake hands/embrace after a match as well, this being a more natural gesture of respect/camaraderie for many people.2
It seems that hand-shaking/embracing after bowing has been the standard in international competition since its earliest days:
But this doesn't seem to be universal between Japanese competitors themselves, e.g:
Is it bad etiquette/frowned upon to additionally shake hands/embrace after a judo match in Japan?
1. The IJF rules specify that contestants must bow to eachother when indicated by the referee before and after competing, but only "The contestants must NOT shake hands before the start of the contest."
2. So much so that refusing to shake hands with a competitor is grounds for disqualification from the Olympics by the IOC.