With every litter this may happen, has nothing to do with rotties...
That is why we use a "litterbox" in order to prevent this...
Look at the box, under the topic "abuelos"... here we already removed 3 of the pipes... but normally in the beginning, the four sides of the box have a pipe about 4 inches from the side and the bottom, so the puppies can roll to safety if mother is somewhat careless...
In the UK, this was known as a 'pig rail' in the birthing pen and, as opposed to a pipe, was a small shelf but functioned in the same way so the puppies would slide underneath out of harm's way. Used for virtually every breed as you say as, unfortunately, accidents can and do happen in those early days.