In general, 20 Ft. Boxcontainers are anything between 2.080 and 2.570 kgs. depending on condition, repairs etc.
Most are 2.50 mtrs. wide
His lack of speed will have triggers the people in blue to go and have a look.
Once there, they may have found a few issues like mirrors etc. too long overhang relativ to the axles etc.
Also, not all 3-axles trailers are 3.5 tonnes.
Nor are all 90's rated to tow 3.5 tonnes.
But as usual with media, most info is missing...