Late 2015 iMac 27 freezes than crashes and restarts - often and apparently randomly
You used a different type of RAM than specified by Apple. It's a small difference, but it is possible that it caused the effect you are describing. You purchased this RAM:


  DDR3L 1867Mhz PC3-14900 SODIMM 2Rx8 CL13 1.35v RAM


Apple's site specifies this RAM:


  PC3-14900 Unbuffered Nonparity 204-pin 1867MHz DDR3 SDRAM


What's the difference?
It's DDR3 vs DDR3L.
The "L" stands for low powered. Standard Apple RAM, and replacement RAM from sites such as OWC, operate at 1.5V. Your DDR3L RAM operates on 1.35V.