Actually all forks can end up with pressure inside, however it is air pressure. As the forks function oil is pressurised as it is forced from one gallery to another, on both rebound and compression. This can cause a partial vacuum behind the oil flow which depending on the location may suck air into the fork through the fork seal. The fork seal by its design only seals in one direction which permits the ingress of air. Once pressurised, the damping may diminish a little, but is nothing to worry about under normal riding. I have noticed oil leaks around the top fork nuts when Dow dampers are fitted but no fork seal problems. And a little sealant stops the oil leak at the top nuts.
Even modern cartridge forks can be affected. My son has fitted aftermarket air bleed valves to his CRF250. After every race he depressurises the Showa fork before the next race.