The difference is that when the borders were still up illegal goods movements were detected more frequently, but most of this waste movement is and was perfectly legal under EU single market rules. The 'four freedoms' explicitly spell this out.

This brings up the whole Multi-speed Europe thing:

https://en.wikipedia.org/wiki/Multi-speed_Europe

Regardless, anyway you view it, these dumpings are a failure of the particular way the current EU has been implemented. There were other options.