Prosecutors believe the driver, who was killed, was not concentrating just before the coach ploughed into standing traffic and burst into flames.
Another 30 people were hurt in the incident on the A9 motorway, near Stammbach in northern Bavaria.
The coach had been carrying 46 elderly people on a trip to Lake Garda in Italy
It was reduced by the blaze to a metal shell.
The driver of the lorry, which was reportedly carrying mattresses and pillows, escaped unharmed.
At the time, motoring safety expert Hans-Ulrich Sander suggested the fuel line that ran under the bus may have ruptured, prompting the fire to spread quickly.
Bavarian Interior Minister Joachim Herrmann said rescuers had been delayed by the intensity of the blaze and by "gawpers" driving slowly.
A horrific bus crash on a motorway in southern Germany last month that claimed 18 lives was caused by driver error, an investigation has concluded.