If after upgrading to OS X 10.11 El Capitan you experience frequent crashes of the viewer, chances are the GPU is restarted by the system logging a message like this in Console (system.log). The issue has also been observed on OS X 10.10 Yosemite
25.10.2015 09.12.39,000 kernel[0]: NVDA(Graphics): Channel exception! Exception type = 0x1f Access Violation Error (MMU Error 2)
Initially I thought it was an issue with a newly built test version of the Kokua viewer, but on further testing both the official SecondLife viewer, Singularity and Cool VL viewer all experience the same crash.
I have found it to manifest itself in areas with high density of mesh items such as the Daley Bay sim in SecondLife, the Kitely Merchant exhibition or local sims used for testing.
While Apple is working on correcting the issue, the fix is to install the NVIDIA Quadro and Geforce Mac OS X driver which will restore functionality. The only caveat is that the increased performance I have observed on El Capitan is lost, presumably because the NVIDIA driver does not support Metal the same way as the native OS X driver.
If your Mac has integrated Intel graphics, installing this driver will not make a difference – if you even were allowed to install it.