When question links are posted in chat (manually or by the automatic system) they get auto-formatted so that the question title and some of the text is displayed.
It would be convenient if the spoiler markup worked in chat, so that chat didn't get unexpected spoilers (especially in the case where the questions are being auto-posted, since a spoiler can turn up in chat with no warning at all).
This is seemingly worse than the text in the question page because more of the text is included, so even if the question text is padded enough to avoid showing the in the question page, it may still spoil in chat. In addition, it's not possible (except for mods?) to edit these postings once they have been made, so once the spoiler is in chat, it's there until enough new messages scroll it off.