No "isolated incident", but the common belief in all armies of the time that a city taken by force (not surrendered), may be plundered for 1 to 3 days by the victors (Praga, Lübeck, Badajoz...)
After the first hours, no adrenalin can explain the savagery of the pillage. Only some weird "right of conquest".
And yes, French did worse. But not against their "allies".