Synizesis is the rare phenomenon where two vowels within a word that normally do not form a diphthong are nevertheless pronounced as such, and hence count as a single syllable in the metre. Under which circumstances does this happen in Latin? Or is it unpredictable?
There's not much literature on this subject -- maybe because synizesis is so rare. Aside from the obvious answer of "whenever the meter requires it", I couldn't find any hard-and-fast rules, just hints.
However, from what I could find, it's particularly common in...
the initial ea- and -eo of eadem and eodem.
words ending in -ea and -eo
the eu, ei and ea in Greek proper names
stolen almost verbatim from A Guide to Latin Meter and Verse Composition by David J. Califf, p.20
The author of the source above gives a few examples as well; for example:
ipse deus vatum palla spectabilis aurea
(Ov. Am. 1.8.59)
among others. See the linked source for details.
Aside from that, though, there really isn't much. A lot of it is very, very technical stuff that I can't quite read, but from what I understand is basically saying the same thing as Califf's book. A lot of the rest is intuitive stuff.