A combination of:
- Irresponsible choice and planning of the finishing areas and corner cutting actions by the organisers (for example, the downhill Katowice finish (at least I believe it was Katowice and don't feel like hunting out the footage to confirm) was already dangerous before the organisers decided that bricks were a good material to shore up the stability of the structure and made it even more dangerous should a rider be flung into them at high speed.
- A péloton larger than would be suitable for this type of finish due to the WT status; these types of finish can often be done with far less trouble in either more selective stages or in races like the Tour of Britain and Danmark Rundt where you frequently have around 16 teams of 6 riding, rather than 25 teams of 7, so around 80% more riders than those other races trying to make their way through
- A race which is lacking in other GC-relevant stages, meaning that there are far fewer riders who are in a position where it simply doesn't matter if they lose time so they don't need to worry about placement coming into the finish. Take the Bury St Edmunds finish in the Women's Tour that had the farcical crashes and the crazy narrow chicane about 200m from the line - they negotiated that exact finish safely in 2014, when it was on stage 5 and in bright sunshine and so only a handful of people were contesting it - those up in the GC and those contesting the sprint (and many of them were one and the same), everybody else could just do their bit to lead out, then cruise back and not worry about placement. This year, it being stage 1 and everybody still having something to protect (plus the weather being worse) meant that far more riders were trying to get into the mix, both GC riders and sprinters, and the finish's dangers were exacerbated as a result. In the Tour de Pologne, frequently the only GC-relevant days are backloaded, so you have everybody still with something to play for trying to keep themselves ahead of any incidents. That isn't all the fault of the organisers - Poland's problem is like that of France but amped up to the maximum - the only terrain available to create a selective race is way over in the south along the borders with the Czech Republic and Slovakia, with a few small punchy hills near Kielce, Szczecin and Gdańsk. If none of those three cities wants to host, you're left with several days relying on the weather to give you anything but large bunch sprints before anything (other than a crash) happens that might reduce the number of people contesting that finish. Larger team size than at comparable races parcours-wise also makes controlling the race easier for sprinters' teams increasing the likelihood of sprints happening even if there are obstacles, if they aren't punishing enough.
Largely flat stage races with only one or two hilly stages can be done in a way that works. However, the races that handle that format best tend not to be races with 175 mostly WT level riders in them, and more races like the Danmark Rundt (with the Vejle stage) or the Quatre Jours de Dunkerque (with the Mont Cassel circuit), which have a mix of levels and often smaller teams. The TdP was at its best when it was able to use either the Krkonose or the Tatra mountains to give us some selective medium mountain stages - not so much that it's "win the MTF win the race", that's no better than the old "win the TT win the race" events like Bayern Rundfahrt and Tour de Missouri, but where you had things like the Bukowina circuits or the old Karpacz split stages which take us back to Peace Race days.