The glass expands and contracts every day until eventually the continual movement wears out the rubber seal which has probably hardened with time anyway. thats why windows facing direct sunlight go quickest.
Once the seal is broken, and it only needs to be a pinhole, the gas inside blows out under expansion but also sucks back outside air when it contracts.
The outside air contains moisture, hence the condensation.
I have been blamed for it as well but its not our fault.