Syllabification of "riding"

It's syllabified as raɪd.ɪŋ because there's a morpheme boundary between the two syllables (the -ing is a different morpheme). Morpheme boundaries can override other syllabification rules and morpheme boundaries are usually syllable boundaries, so it's syllabified that way.

According to Prof John Wells:

In polymorphemic words, consonants belong to the syllable appropriate to the morpheme of which they form a part. This applies only to synchronic, psychologically real morphemes.

[Wells Syllabification - UCL]