I opened the file and it was in shading mode. It took a while for shader compilation while the GPU was busy. Then everything became normal, memory ~350MB, CPU ~7% and GPU idle. Switched to solid mode and the CPU/memory activity is still the same. GPU is also idle unless I move the object, which is expected.

There's nothing out of ordinary. You can check Activity monitor > window menu > GPU history to see if GPU is working. Check the temperature using Intel Power Gadget. Below 95 C seems to be fine.

Darwin 18, Intel HD 6000.

I also continued working with the same file and can't get it to repeat the overheating this afternoon. It happened this yesterday morning 3 times and has ghosted me two other times. Perhaps it's just the particular brief activity / setup I have going that sets the process in motion. I have been using EEVEE occasionally and that shows little if any activity in the CPU and none in the GPU. The SOLID view, doesn't show much that would indicate the overload although I don't have time to turn on Activity Monitor before shutting down the overload--it's that fast when it happens.
I assume I am the only one to experience this and will assume that it too shall go away with the constant Blender updates. I really thought I had it nailed. If no one else finds it in a week then mark this "resolved" again and I'll just be careful until I can get a few versions newer.....
Thanks Ankit for the effort!

I'd guess that some other background process like Spotlight was busy at the same time as Blender. but that won't explain the repeatability thrice.
Do you have multiple GPUs ?

although I don't have time to turn on Activity Monitor before shutting down the overload

I keep it open all the times with dock icon showing CPU usage.

Yes, I'll keep Activity Monitor on all the time using 2.9 now. I only have the one GPU:
Radeon Pro Vega 48:

Chipset Model:	Radeon Pro Vega 48
Type:	GPU
Bus:	PCIe
PCIe Lane Width:	x16
VRAM (Total):	8 GB
Vendor:	AMD (0x1002)
Device ID:	0x6869
Revision ID:	0x0000
ROM Revision:	113-D0650E-072
VBIOS Version:	113-D05001A1XG-011
Option ROM Version:	113-D05001A1XG-011
EFI Driver Version:	01.01.072
Metal:	Supported, feature set macOS GPUFamily2 v1
Displays:

iMac:

Display Type:	Built-In Retina LCD
Resolution:	5120 x 2880 Retina
Framebuffer Depth:	30-Bit Color (ARGB2101010)
Main Display:	Yes
Mirror:	Off
Online:	Yes
Automatically Adjust Brightness:	No
Connection Type:	Internal

...so it has to be either the CPU or GPU overheating I assume ... I've seen the GPU jump to 92% briefly. IF no one else has experienced this (I'm surprised) then it must be a quirk of the current OSX environment (with the event cascading) that is the cause and MS is saved (this time ; > }}