Yep. It's as Charlie-San says and a bit more complex than that.
It's not just for the simple reason of being busy with other stuff. You have to be an author or familar with the concept in order to understand it. It's perhaps because the mangaka is tired, dry, want a change of pace, got annoyed with, ran out of ideas, find it tedious now..etc
The same thing happens with authors of books (i read fantasy and that's why i'm familiar with it).
The reason he's doing his other mangas is because it helps the mangaka unwind. Perhaps writing Ubel is not fun anymore. Maybe he couldn't just handle the deadlines. Maybe he can't keep up with the weekly schedule. The point is, and keep in mind that these are all speculations, the mangaka needs a change of pace.
In chapter 98 (the last published chapter) at the last page it mentions that the next chapter would be on November 2009. But you know, shit happens. On his blog he just says he's gonna take a break.
The same thing happned to J.KRolling. I don't remember the exact details, but after releasing a book (could be 4 or 5), she said she's gonna take a break and enjoy life. She released the next book 2 years later.
So, we are seeing the same thing with SHIONO Etorouji. He wants a break, and apparently, he likes writing gender bending stuff. Go figure.
I can guess that we could see Ubel back in the end of the year, but who knows. I dont have any info, and i always tell people to go troll 2chan and see if they can find something out.
Also keep in mind, Ubel is not your typical manga. Doing a manga with that level of details is not easy on a weekly basis. I've seen the quality drop on the mag scans (some minor details that he couldnt finish up by the deadline), only to fix some of it on the tank release.
The same thing happned with the magaka of Vinland Saga. He got burned out, and now he release it on a monthly basis (this is my logical guess, nothing i read anywhere).
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p.s: ch96 is out