I thought random events would only target active pet (the hover text in the pets page said "Your active pet is used by various features around the site as the pet that has things 'happen' to it. Be careful who you choose!"), but I just got an event

"Whats this? Cookie's treasure chest has grown 1 space!"

But my active pet is Icarus. Is this expected?

Yes, random events can raise your other pets' treasure chest but with other stats, they do go to your active pet if these is a random event.

Ah so treasure chest event is an exception...I see, though I'm also wondering if there's a reason?

I don't know why but they are coded in thius way.:P May be others know about it? But since it is not a bad thing to ppl that your pet's treasure chest is raised so they just simply don't care?:P