Sounds like Ozone cracking, how deep are the cracks? if it's just surface crazing then don't panic but you need to think about getting them replaced as they have started to deteriorate.
Does the van stay stationary for long periods? When the compound (rubber) for your tyres was mixed, an ingredient called an "anti-ozonent" was added, usually in the form of waxes.
When your tyres are in use, the flexing and warmth generated brings these waxes to the surface where they form a barrier and protect the compound from ozone in the atmosphere. If your van stands unused for long periods then this does not occur and the compound comes under attack from ozone.
Ozone is NOT what you smell at the seaside (that's rotting seaweed) Ozone is a poisonous gas which is formed by the action of UV light on Oxygen and is very harmful to rubber compounds (and humans if you breathe enough of it

) fortunately it only forms in very small concentraions naturally, not strong enough to harm you, but strong enough to harm your tyres.
The solution? get out in your van more

but if that's not practical, protect your tyres from sunlight (UV) by either covering them or regularly applying wax - boot polish for instance. I'm afraid that the process is irreversible though once they've started cracking they won't "come back".
If on the other hand, you use your tyres regularly and you are getting ozone cracking then there is something wrong with the compound, contact the makers and explain the problem to them. This is assuming that the tyres are from a reputable manufacturer - unfortunately some of the bargain basement companies do not go in for expensive product testing and cut corners wherever possible to keep prices down, you are unlikely to get any joy from them.
Sorry for going on and on and on, I got carried away ( I used to do ozone testing as part of my job - did you guess?

)
Frank