In Friday's Asian trading hours, with western markets closed, WTI prices suddenly plunged through both the $45/barrel and the $44/barrel marks. WTI contract volume skyrocketed from the average of hundreds to the more than seven thousand in minutes as market participants were forced to liquidate positions. The exact cause of the sell-off is unknown although analysts and traders strongly suspect that recent events coupled with a blend of margin calls and algorithmic trading were to blame.
The rise of computer trading has brought significant scrutiny and publicity to what was eventually termed high frequency trading. However, among the many flash crashes in commodities, equities, and currencies that have been observed over these last several years, usually some form of human error triggered a whale's (large market participant, say a hedge fund) algorithm to sell, which in turn led to other algorithms to sell. Flash crash bottoms usually form at circuit breakers or when human participants realize prices have reached irrational levels.
In-depth discussion with some technical details: https://www.bloomberg.com/news/articles/2017-05-05/five-charts-that-explain-crude-oil-s-sudden-nosedive-toward-45
