I think Dragonboy strikes a good point. While DK's update might seem lame, it is a really fitting one. I mean, sure, people would expect something like a coconut gun, but you know what? That's not really DK specific. Seriously, replace the coconut gun with the cracker launcher, and you get the same idea.
See, if you look at each character so far having a BACON, you can see why they got that one.
Mario has a fire-based BACON, which is a logical choice, since most of his old attacks were fire-based. In fact, Mario is often associated with fire.
Link is always associated with his sword, so something involving a sword would be fitting. However, to give it a Zelda theme, he also has a Triforce appearing.
Kirby's is obvious. He pulls out a hat, which is something very Kirby, and uses his cook ability, a very rare ability in the Kirby games. Now, this one needs some quoting from Wikipedia:
"Practicing his culinary skills, Kirby throws every enemy on the screen, even defeated mini bosses, and cooks them, turning them into scrumptious food that restores health. This ability is rare, and is only able to be used once. It appears in Super Smash Bros. Brawl as Kirby's Final Smash move."
So it could be that this attack can suck in enemies. In Brawl, this could mean your opponents, or at least one, will be sucked in the pot.
http://en.wikipedia.org/wiki/List_of_Kirby_powers
Pikachu's BACON is a less obvious, but in the end, you can understand why he has this one. Volt Tackle actually is a move learned by Pichu, however, since Pichu can evolve into Pikachu, it was still a logical choice. Interestingly, Pichu could only get Volt Tackle when breeding with a Light Ball.
http://www.serebii.net/emerald/volttackle.shtml
Bowser can be a logical result of Giga Bowser's appearance. This might come from the cutscene in Super Smash Bros. Melee, when facing Giga Bowser in Adventure Mode.
Samus' BACON is the result of overcharging a beam, thus a logical choice.
So, if you deduct from all of this, DK using his bongos is the logical result of DK's persona.