Short explanation:
Companies in Poland are taking garbage for money (including toxic and/or this difficult to store) from all over the Europe and store it in Poland. They are doing that for the cheapest price and don't care at all about storing it in the right way. As you can see from the map below, those places are often cities based near highways and this issue is more common in the western part of Poland, because it is much easier to drop those garbage there while driving back from other EU countries. This is more common now because of China's ban for garbage import.
Why are there so many fires? Often those landfills are not designed to store such garbage. There are also speculations that mafia could be involved. Looks kinda like a domino effect, owners of those landfills can be in panic and burn the whole places down (Speculation)
List of articles which are explaining everything - I'm sorry that every one of those are written in Polish, but I've found just one article in English about this problem. I've checked with google translator and it looks ok, you can ask me in the comments if something isn't clear.
For more info please go to original article: