We've been seeing a similar issue in Ohio with our apiaries (bee hives). For us it has been three items. I can image it's the same in Europe:
* Varroa mite - a Chinese mite that spread to Europe and then the by cross species breeds. It bites into the side of a bee. Adult bees are usually strong enough withstand the bites. But the mite's feeding leaves holes in the shell that never really heal and makes it easy to contact diseases like Fowl Brood Hive Disease.
When the mite hit the brood (aka larva) they'll grow up in cell with the brood. Regular brood will have the mites cleaned off or the brood and mite larva will be removed as soon as possible. But drones have a longer gestation period and deeper cells. By the time the drone emerges the mite has already gone through it's full life cycle and is ready to restart the process attaching to nurse bees. Ohio would have eradicated it by now but neighboring states don't regulate their hive.
* Fowl Brood Hive Disease - It's bacterium that spreads on contact between bees. Since bees are very social it spreads very easy. Remember when a bees stings something it dies, so why fight when you don't have to? Plus queens prefer to breed with other hives to prevent genetics stagnation.
When a brood has been infected it forms scales on it's shell and begins to for scales similar to Harlequin Ichthyosis in humans. If they die in the cell the nurse bees will remove the decaying brood. In the process the nurse bees also get the spores on them spreading the disease in them. At the point the only good way to stop the disease is to kill the whole hive, clean out the hive, and boil everything. Again Ohio would have eradicated it by now but neighboring states don't regulate their hive.
* Genetically altered crops - We have enough floral diversity in our wetlands, open fields, forests, and private gardens to feed our bees. But we sees build-up of genetically modified pollen that has a pesticide in it. The first couple years of a hive theirs no issue. But around year 3 we can almost count on total hive collapse. Its because the pesticide builds up in the wax they keep rebuilding for the brood, nectar, and honey storage. Eventually to get so strong it poisons the brood or the hive swarms to find a better location.