Thiers again -

The two columns of the Army of Italy ... one commanded by MacDonald, ... were ascending the plateau to join Dupas when, perceiving the latter's Saxons, and taking them for enemies, they fired upon them. The Saxons, thus unexpectedly attacked in the rear [remember, these are Saxons under Bernadotte's command, being attacked by troops under MacDonald's command!], returned the fire, and the troops of MacDonald and Grenier, believing themselves charged in front [by their Saxon allies, whom they had been firing upon!] while at the same time they were attacked in flank ... by Hohenzollern's corps, were thrown into a state of confusion, which the night soon converted into a panic. They rushed down from the plateau, followed by the dismayed Saxons [whose position they had effectively destroyed!] and took to flight in incredible disorder.

Oh, yes, obviously Bernadotte's fault.