My guess would be that the van was repainted and the rust was painted over and now is bubbling again.
not too bad considering the age of the van, especially for a Sprinter! (the roof paint is generally meant to be terrible on those)
What I would do (and maybe will on mine as I have a few bits of rust on my seams) is the following:
1) sand off the paint on the seams to expose the rust
2) apply some
Hammerite KRUST. This does a good job of stablising the rust (it claims to convert the rust back to metal I think but that is a bit of hype). After a short time, the area goes from blue to black as it works, and then you can overpaint.
3) overpaint with solvent paint, not water-based (for old vehicles like ours are, I go for Cellulose-based paint that doesn't need any top coat)
As an example of using KRUST, I did the following on my 2005 RAV4 I bought in 2015
The fuel filler pipe is a known weak spot as it is behind a cover that fills up with wet mud that compresses against the pipe
WP_20150517_020 by
David, on Flickr
I really didn't want to replace the pipework and it wasn't actually holed yet, so painted in in KRUST
WP_20150517_022 by
David, on Flickr
Blue when it is applied and reacting
Then as it finishes reacting and dries, it goes black
WP_20150517_024 by
David, on Flickr
In this case, I just painted it when dry in Hammerite paint - in your case, you would use solvent paint as it is visible
WP_20150517_030 by
David, on Flickr
Bottle of KRUST is under a tenner and lasts ages.