I have lived in a few premises with block paving, and the first one the blocks sank, and two blocks not moved all treated the same. As to if the first one could be blamed on cleaning who knows, and this is the point, it is impossible to say why a drive has failed, I know the last drive the people laying spent ages with the preparation before the bricks went down, and there was no sand under the bricks the sand only filled gaps after laid, but the one that sunk had sharp sand under the bricks.
Now if the person laying it has given instructions that the drive must not be power washed then he would have a good case saying the fault is due to abuse, today main reason for brick sets is so water drains through the bricks, but years ago I have seen them laid on dry lean, which will set as hard as a rock and would not let water drain through it.
So I am sure the drive where bricks moved it likely would not have moved had it not been pressure washed, but also it would not have moved if it had been laid well in first place. However as soon as you see there is a fault to continue to pressure wash is clearly wrong, you know there is a problem, but until they move you simply don't know. So damage to date would be hard to blame you for, but if you continue then yes your fault as you know they are not well laid.