What forces expelled these huge clouds, then blocked further progress, yet allowed it to maintain its threads?
This was a coronal mass ejection.

Those 1973 astronomers weren't looking at the picture correctly. They didn't have the tools at that time to look at the picture correctly. Coronal mass ejections (the term used now) were only discovered a couple of years prior to that picture taken from Skylab, via the Orbiting Solar Observatory 7 satellite. Those early 1970s astronomers called such events "coronal transients". OSO-7 took 45 minutes to transmit a single 256x256 image back to Earth. A proper look at the picture took a few more years to develop. Those coronal mass ejections (the term used now) eject massive amounts of solar material into space.

When looked at via a single still image, it does indeed appear that material is looping up only to fall back to the Sun.



Image credit: SOHO (ESA &amp; NASA)

A better view of a coronal mass ejection became available once those satellites had the ability to send more than an image per hour back to Earth. The image below portrays a coronal mass ejection that occurred in 2001.



Image credit: SOHO (ESA &amp; NASA)