NEW YORK, Aug. 29 -- The traders who spent July betting against bitcoin were right for about four weeks. Then roughly $4 billion of their positions were closed for them, at prices they did not choose.

Bitcoin changed hands near $80,253 on Friday, up close to 10 percent over the week and back at a level it had not touched since May. It reached $81,237 on Tuesday before stalling. The stall is the more interesting half of the story.

What happened in August is a clean reversal of what happened in July, and the disagreement now is about the engine. One reading is that institutional money came back and kept coming, eight sessions running. The other is that the move manufactured its own buyers by forcing bears to cover. Both readings are suppo...