Yeah, the last few weeks of Kai in Japan were pretty crazy. Apparently, Toei found that what little work was actually put into producing the series wasn't cost effective (it would seem that the ratings were there, but that the merchandise just wasn't bringing in the money they were expecting).
There was never even really an official cancellation notice, either. Toriko just replaced its timeslot. And due to the earthquakes in Japan, the last episode (where Trunks returns to his time to defeat the Androids and Cell) never actually aired.
As for the music replacement, the composer of Kai's soundtrack, Kenji Yamamoto (who had been doing videogame music for the series for quite sometime, as well as a few songs for the original Z portion of the series) admitted to copyright infringement... Something that the Dragon Ball community has joked about for a while (for instance, Cell's theme song in one of the old Super Famicom fighting games is almost exactly Pink Floyd's "One of these Days").
As you can read
here, there are numerous instances of this. The popular theory is that there could be many more infringing pieces, and that Yamamoto himself may not even be able to list them all (many have given him the benefit of the doubt, and assume that he simply took a little too much inspiration from certain pieces, rather than just copying them intentionally). As a result, Toei chose to toss the whole thing out.
This actually happened when there were only two or three episodes of the series left to air, though the remaining home releases had their background music replaced.
Though, with the way these releases were tiered, it has resulted in some episodes having different background music for certain episodes in the Japanese Blu-Ray release and the Japanese DVD release and the US domestic releases (there's a handy chart on the link I provided that illustrates this).
The replacement with the original score also wasn't handled super well, with some piece selections not fitting particularly well with the scenes they're paired with. I suppose they had to rush to replace the soundtrack, since the series was running in repeats on Japanese satellite stations as all of this was going on. I have to assume that FUNimation was simply given versions of the episodes with the new-old soundtrack for volume five and onward.
It's all very weird. I'm actually not a big fan of Kai. I was interested in the initial idea, but thought it was executed pretty poorly. That said, I found all of the end-game stuff surrounding it to be really interesting.