Intergraph's workstation business died before SGI was in real trouble. Intergraph had spent $1B on their workstations and wound up with a buggy (due to Intel) machine that went for $20k, while at the time most people could get an Octane for that same price. The only people who bought Intergraph workstations were people who bought Intergraph software, so there wasn't any overlap in markets through the 90s. In fact, we often used the Intergraph example as why you should buy an SGI workstation.
3dfx didn't run OpenGL at all. 3dfx ran Glide. At the time, Glide was far superior to OpenGL because it was almost direct to hardware, vs. OpenGL's abstraction layers, which further complicated things since game developers often assumed that a rendering feature was supported by hardware, when it wasn't. Games were written for Glide. 3dfx had 80-85% of the 3d accelerator market from 1995-2000. It was 3dfx's capabilities that opened the eyes of our team and all of our customers that PCs were ready for a lot of what O2, Octane, and even Onyx computers could do. (Search on the Voodoo and the Voodoo2.)
Windows NT, launched in 1993 while SGI sales were still rising. If anything, the work in porting from Irix to WindowsNT caused people to stay with Irix. You could list NuTCRACKER, released in 1998(?), was a catalyst, but NT itself was not.
Pentium Pro was a good chip - that's why SGI used it in some of their low end server offerings. However, the Pentium Pro suffered from a hobbled floating point pipeline - running about half as fast as MIPS and SPARC risc cpus. So, the Pentium Pro did not encroach at all on SGI's core business.
NVidia was there to pick up the pieces, but at the time it was one of many PC graphics card manufacturers. NVidia didn't have a market leading card until the TNT/GeForce releases in 1999/2000.