Mistake #4: Broadcasting
It's generally considered bad form to transmit with no expectation of a reply. The APRS beaconing is marginal, but it's not eating up much bandwidth and 144.390 (on 2m) is universally accepted as a dedicated frequency. Another accepted 'broadcast' are HF beacons that people use to determine propagation.
It's also possible since you were moving that you came into range of their conversation. You are supposed to listen a while and check that a frequency is not in use before transmitting. But if they were mobile they could be guilty of that, too.
"Why does the U.S. care which flag will be hoisted on a small piece of land thousands of miles away?" -- Ron Paul