Some miscellaneous notes about possible causes ...
There was a cluster of comparatively frequent geomagnetic field reversals from circa 22 Ma to circa 17 Ma, and one such shift occurred right at circa 19 Ma.
The early Miocene was the timeframe for a sustained global warming trend (starting at around 21 Ma) that reversed the longstanding trend for cooling worldwide termperatures. This warming trend was well underway as of 19 Ma, and it would continue until circa 14 Ma.
The early Miocene was the timeframe in which ocean kelp became prominent and radically proliferated. Kelp forests may have made it easier for prey fishes to evade sharks.
There may have been a major impact event during this timespan. The Karakul / Kara-Kul impact in modern Tajikistan has been variably dated to a wide range between circa 23 - 25 Ma maximum and circa 5 Ma minimum. This is the only impact structure so much as suggested to have occurred around 19 Ma.
Both sharks and cetaceans grew in size and diversity in the early Miocene. In particular, a number of predatory cetaceans emerged and proliferated during this timeframe. Some were big enough to compete with, and conceivably prey upon, sharks.
