an infelicitous implosion

There are roughly 200 English words ending in the letter sequence <-sion> - from abrasion to vision. This ending is pronounced either /-ʒ(ə)n/ or /-ʃ(ə)n/. Is there a rule behind this? Look at this list:

abrasion - accession
adhesion - aggression
collision - aversion
conclusion - convulsion
decision - declension
elusion - emulsion
implosion - impression
infusion - intension
lesion - mission
etc.

The first word of a pair always has /-ʒ(ə)n/, the second always ends in /-ʃ(ə)n/. The rule is quite simple (at least theoretically): If the letter preceding the <-sion> represents a vowel, it's always ezh (or yoghurt if you prefer the latter term), a consonant letter (silent or not) leads to /-ʃ(ə)n/. (See also Jack Windsor Lewis's website, section 4.5).

Listen to Canon Tilby's way she pronounces the word implosion in this sentence:

"If when an invitation comes, you find yourself scheming your way to turning your fantasy into reality you run the risk of implosion."

 None of the big three pron dictionaries records a variant with esh.