If you had to pick an inflection point where Sun's dominance in the Unix world began to decline it would be this.

Not in terms of total installations, momentum and exponential expansion of the entire industry distort that, but in terms of it's influence. This is also the exact same time that 386BSD/NetBSD/FreeBSD and Linux started releasing.

As others have pointed out BSD-based SunOS was the king of Unix-like operating systems in the late 80's to early 90's. Sys-V based Solaris (1992) was an immediate flop and quickly earned the nickname "Slowlaris". Only by version 2.4 was it even usable in production. By 2.6 it was a decent operating system but anyone who remembered using SunOS wished they had never switched.