Learn Finnish: Epäilyttävä ääni häiritsi heitä kaikkia yövuorossa. - A suspicious sound disturbed all of them on the night shift.

This is the core of compound pronoun phrases! When you combine a personal pronoun like he and a quantifier like kaikki, they must agree in case. Because the verb häiritä (to disturb) always requires a partitive object, both words take the partitive ending to become heitä kaikkia (all of them).

Could I say kaikkia heitä instead of heitä kaikkia?

Yes, the word order in these compound phrases is flexible. Kaikkia heitä is grammatically correct and means the same thing, though putting the personal pronoun first as heitä kaikkia is generally more common and natural in Finnish.

Since the disturbance happened in the past and is a completed action, why doesn't the object take the genitive-accusative case?

Häiritä is an inherently partitive verb. This means its object must always be in the partitive case, regardless of whether the action is ongoing, completed, or happened in the past. The verb's own rule overrides the general object rules.

How is the adjective epäilyttävä (suspicious) formed?

It is the present active participle (the -va or -vä form) of the verb epäilyttää (to cause suspicion), which itself is derived from epäillä (to suspect, to doubt). The participle acts as an adjective literally meaning 'causing suspicion'.