They were probably allies of Rome’s to begin with. The Romans occupied Lowland Scotland, the Emperor had other priorities than finishing up the conquest, so the army built the Antonine Wall. Later they built and pulled back to Hadrian’s wall. Still later they reoccupied Lowland Scotland and reconstructed parts of the Antonine Wall. Gosh, it’s like modern politicians were in charge...
Modern politicians in our own country can't even build a contiuous border fence to keep out the barbarian hordes.