It seems this may have something to do with unlocking Zereth Mortis flying. That’s approximately when it started happening to me, and now that my husband has unlocked ZM flying, it’s happening to him as well.
When I log in on any of my characters, the “collected” sound and animation is displayed–the glowing gold burst of light, and the sound as if they’ve just learned or “used” an item, like a pet or mount. This sound and graphical effect happens to every character on my account, even brand new ones. It also happens on my other, inactive, WoW license.
Interestingly enough, I just logged in to my account on my husband’s computer, and it still has the learning sound and animation on my characters when they log in. However, it doesn’t happen on any of his characters on his account on his computer.
I have done all of the troubleshooting steps outlined in this post in Tech Support: Learning sound and animation - #3 by Kershew but the issue persists. I opened a ticket as instructed, and was told to file a bug report.
It seems this may have something to do with unlocking Zereth Mortis flying. That’s approximately when it started happening to me, and now that my husband has unlocked ZM flying, it’s happening to him as well.
There is also someone else in the EU that’s having this issue (they posted in TS in the EU forums), and though they haven’t connected it to ZM flying, they did note that the issue started happening on 16 March, which would be about the right timing.