I'm posting this here so I don't have to keep explaining it indivually.
I don't think this happens much anymore, if it all, so it's mainly historical.
Today, for a major show (ie. Lucky Star, Sola, etc.) subbing by a major group typically happens very fast, a matter of days after the show has aired. Used to be, even with a major show, it could be weeks before you saw a sub come out.
So what would happen is like this. If up through episode 8 of a show has aired, and your group has been with the show from the beginning, releasing 1, 2, and 3 in the past, is working on 4 in the present, and in the future plans to continue the show.
At this same time, a rival group with the intent to 'hijack' the show starts working on episode 7. They're not as good or efficient as your group, so it's going to take them time and a half to complete it.
Meanwhile your groups completes and releases 4 and 5. Your rivals finish 7 and don't release it yet (since nobody has released episode 6). By the time you finish 6, your rivals have finished 8, but again, they don't release it right away.
So the second you release 6, they release 7 and 8. The fans don't really care, they'll download your 6, someone else's 7 and 8, and be extra happy. Your group can probably get the fans back 9 or 10, but by then it may be licensed or the damage is already done.
Not to say this kind of thing couldn't backfire on your rivals. The show could be licensed before they get a chance to release the episodes they've been holding back. Or your group could take its own countermeasures if they know if the impending hijacking attempt. Upon completing episode 6, hold it back and finish 7, 8. Then release 6, 7, and 8 all at the same time. Your rivals just did all that work for nothing.
Some groups were really paranoid about "getting 'jacked" and constantly trying to spy or infiltrate their rivals to figure out the episode numbers they were sitting on. If a group openly denied they were working on a series, especially one they had said they were going to do earlier, some of the more paranoid groups would taks this as a de-facto admission they were planning to hijack the series once a few episode were out.
I don't remember when the practice ended and the paranoia went away. But it wasn't very common to begin with; it was mainly the paranoia and idle threats or bluffing made by troublemakers / agitators. These days, it would be near impossible since when it comes to a popular show, the next episode isn't even going to be avaliable in raw form before the current one is released.