Not just Leons, all cars on the Golf IV platform. The self-adhesive closed-cell foam strip breaks down with age and starts to leak. Either no long-term testing was done or it wasn't effective.
The main failure mode seems to be brought on by a combination of age and mechanical shock. The driver's door is always the first to go, and it is the one that gets used most often. The replacement is a non-setting, solid sealant, so I guess somebody in VAG Engineering got a rocket for choosing a material that turned out to go porous with age.
Not cool to wish anyone a slow and painful death, though.