The self-protection works on a per channel basis. So if there's a loud drum that exceeds "spec" on one channel but not others, the protection for that channel will activate.
Now you say it wasn't loud but you may have been listening to a dynamic piece that took the signal say 22dB above nominal that activated the protection.
That protection by the way is very transient. If it was stuck off and you could only clear it via the power switch, there's something else going on. FWIW, when the protection kicks in on my Pioneer, I have to cycle the power. Maybe there are certain conditions that activate that same kind of behavior on the ADA. Or maybe you have an older ADA and the protection on the new ones behaves differently than yours.
