Well, if you remember from the OAV/Movie, both Valks were designed to go into the "Special Forces" role (get in there stealth-ily, drop whatever weapons you've got, and GTH out), then to transition to the frontline "everybody gets one" role. The -19 and the -21 showed that they were really able to do it, despite the crazy pilots that were onboard.
also, from the OAV/Movie, they got picked up as soon as they came out of Fold, but with them having an "allied" signature, it probably didn't think anything of it until they blasted the satellite structures for cover. It was then that they started getting fired upon.