@lexzphantom
It depends on a case by case basis.
a) In 5toubun's case, LHT's translation was so bad to the point that some anons translated chapters themselves and eventually they took over the manga.
b) In some cases a group has a slow release schedule even though the manga they handle has good translations. It might be because of real life commitments (translators are people too), lacking staff to churn out chapters, or some other reason. Then some fans of the series try and translate it themselves to hopefully catch up to the raws (we're about 30+ chapters behind and what's happening in the manga right now is great).
Well some translate it just because:
i.) they really want to catch up to the series and want other people to become fans of the manga (especially since Juliet just finished airing its anime so new fans will try and read the manga but might get discouraged if translations are behind by a lot).
OR
ii.) to make the original group speed up their translations. Sometimes it works, group A (in this case dropout) releases chapters too and then group B (anons) stop scanlating after they see that group A is really trying to catch up to the current raws or group A and group B team up and translate the series. Sometimes it doesn't, group A see this as sniping then scanlation drama happens (don't even get me started on this shit).
OR
iii.) outright trolling, they hate the group or whatever reason they just want to go with.