As a result of dumping delicious candy on Black Bird's house, Black Bird responds with what many would widely consider to be a rather disproportionate revenge - she's mailed what appears to be a small ticking box to Rainbow's house in Cloudsdale, with the instruction to glue it directly to her Wonderbolt uniform. Sure the whole candy thing can't be traced back to Rainbow, but Black Bird knows she's behind it. She does.
I mean, all you did was crush her house under a huge pile of citrus sweets. A bomb's a bit much. Probably. Justice is relative, after all.
Unfortunately, despite the best efforts of the local authorities (the bomb disposal dogs were foiled as everypony remembered dogs are green and red colourblind) and so it looks like you're going to have to do it yourself. Using only your rudimentary skills in bomb disposal from the lessons at Fancy Pant's last dinner party (don't ask, the tale is too disgusting for small ears), you set about the task.
Fortunately, Black Bird doesn't appear to be completely without a sense of fair play, so attached to the bomb is a note, saying that you can only disarm the bomb by following chess and an intimate knowledge of queens. Chess is a boring game for eggheads like Twilight, so it should suit you perfectly, Rainbow reasons. Gee thanks, Rainbow.
After studying the bomb for a bit, you realise that in order to shut it off, you need to hit the squares on the grid, which is shaped and coloured exactly like a chess board, to place eight queens on the board. However, absolutely none of the 'queens' are to put anywhere on the 'board' that would allow each other to be taken, if one were to follow standard chess.
Despite the fact that this stinks of a trap, you set about disarming the bomb. Unfortunately, you spent so much time studying it that you only have about ten seconds to enter the solution and hit the switch to disarm. Move those hooves, rainbow's place of residence depends on you!