You have to be constantly vigilant and defensive. It doesn't matter if you are legally right but dead. In your case, although technically you did nothing wrong, it sounds like you might have stopped within the truck driver's blind zone. If he had hit you, he would have been to blame, but that would be little consolation if you were seriously injured or killed.

I had a similar situation in my car one day. I had just parked along the street and pulled up about 5' behind a pickup truck that was sitting there idling. The truck had a full load, that apparently obscured the driver's vision. For some reason, he started backing up but I had already shut off my car and couldn't move. I honked my horn, but he kept backing up and hit me. I have no idea why he suddenly decided to back up but could do nothing to prevent the collision. Fortunately I wasn't hurt and the damage was relatively minor. The driver acknowledged his fault and his insurance company paid for the damage to my car. I'm just glad that I wasn't on my bike.