A news report published at the time claimed the bridge was blown up by retreating Romanian troops. Not quite true. Another account:
The destruction of the bridge over the Borcea Arm
The firm intervention by General Averescu saved the Cernavodă Bridge from being destroyed, but the advance of the Central Powers troops across the Danube had to be stopped. For this purpose, it was decided to destroy the secondary bridge, namely the section that crossed the Borcea Arm, which was rigged with demolition charges by the withdrawing Romanian troops, in order to prevent the advancement of the German-Bulgarian troops. The Borcea Bridge was rebuilt in 1921.
I regard the following report as unconfirmed but it does name names which may be helpful. Undated, it appears to be from the Second World War.
https://books.google.com/books?id=qO...bombed&f=false