Great question that I cannot answer but looking at these posts it seems pretty cronic for Vw's compared to other makes/models so I must guess their use of connectors and angles as well as drain tube diameter are to blame.
Usually these happen with cars that spend their life outside and especially around trees. In the case of my T2 it's been a garage queen so if it's in fact due to dirt...bad design must be to blame.
Yeah my wonder when I read these threads are...does this car see a lot of trail time? Is it garaged? Parked under trees? Etc.
I don't want to tempt fate but having many Audi/VW cars over the years (almost all parked in the elements their entire lives) I've never had this problem but have always seen threads like this for all the different models I have owned.
Just makes me wonder why I haven't had the issue others seem to have.