Because the root word is two syllables, so you can't lose that existing division. It already divides between the M and the B, so the B is pronounced there too:
So the syllabification/spelling rule that applies there is only that you drop silent E before adding ER.
Words that end in -ling
are a little odd from the point of view of syllables, though, because some people will pronounce them colloquially almost as if the L were a 3rd syllable in the root word:
There's a song about the tumbaling (sp?) tumbleweeds that actually sets the word 'tumbling' to three notes of music (IIRC), but it's meant to mimic a cowboy accent.
In answer to your question, here's an older thread with a link in it that I found to a long discussion of the finer points of hyphenation/syllabification. It wasn't perfect, but it was pretty good. There are definitely some AE/BE differences, though they tend to be diminishing.related discussion: Silbentrennung / Hyphenation
The easier answer, though, is that any monolingual English dictionary will show syllabification of all entry words and related words. Computer spell-checkers and hyphenation programs have algorithms that follow these hyphenation points. So when in doubt, the easiest thing is just to look it up, or to select a section of text, let your word processor hyphenate it for you, and look at the result. If you do that regularly for a couple of months, you'll probably start to notice the most important patterns.