It doesn't do it normally; only when present in a macro that fails to cast for whatever reason.

At least, that's what WoWWiki says. Lemme find the page.

EDIT: OK, apparently I took one bit of info and applied it elsewhere. The event I described is only listed as occurring if you have mutliple "/cast" commands in one macro.

In general, you cannot cast more than one spell with a single click of a macro. Most spells and some items trigger the global cooldown (GCD) which keeps you from taking too many actions at once. Even if a spell fails to cast, if it would trigger the GCD, it prevents subsequent /casts in the macro from running. This was not the case prior to patch 2.0 which is why you may still come across macros like the following:

/cast Overpower
/cast Execute
/cast Mortal Strike
/cast Sunder Armor

Macros like this do not work anymore.
From WoWWiki.

Also, I removed the quotes from this thread. Forgot I used that button to reply.