Canso, when the bottles are bulging like that, it's likely that it's not because of gas pressure alone. With 20% head space, the vapor pressure of liquid propane is less than 200 PSI at all temps <100F (38C). The BIG concern if you overfill and don't have much (or any headspace), is the expansion of the liquid as it warms. Unlike a gas, the liquid is not compressible and it can easily build pressures of 100 atmospheres or about 1400 PSI. The worst possible thing one can do is to completely fill the container with cold liquid (made even colder perhaps by cooling the container) and then let it warm. I saw the result of that when knucklehead in my chemistry department did that with liquid ammonia. He filled two small "lecture bottles" (gas cylinders about 2" in diameter and 16" long) to the brim with liquid ammonia at liquid nitrogen temperatures. He placed the bottles in a hood and left for lunch. Lecture bottles have a side wall thickness of about 1/4-3/8" in steel. Both bottles violently exploded and ripped 3-5" gaps down the side. Fortunately, no one was injured. I later painted one of those bottles gold, mounted it on a nice piece of lucite and gave it to him as an "award" (for stupidity) at a department banquet.
My suspicion is that your pressure relief valves were stuck due to age and corrosion, cold temperatures (the can cool when filling) or a combination of both. As a result, the containers were overfilled and the liquid expansion generated pressure high enough to cause the bulge. IMHO, if you haven't already, you should empty those cylinders and dispose of them immediately. .
