No idea, nothing unusual in size, shape, weight of his bag- all 100% normal. The same bag & contents recently having been checked in at T1, T3, Tallinn, Helsinki, & Dublin with no issues. However, Qatar at MAN for some reason were directing a lot of people this way (my suspicion was actually a problem with the check in desk baggage belt at the Qatar desks).
Normally the extra bag drop desk is for bulky shape luggage- but this was not the case on Saturday.