Why would uninitialized SD-card module without card disable CAN-Bus shield? (I am able to successfully initialize CAN-Bus shield and SD-card module w/ card in it.)

Now I have removed all code related to the SD-card module, yet I left the module (without the card in it) physically connected. This results in the CAN-Bus shield initialization failing.
When I insert the card into the SD-card module the CAN-Bus shield initializes successfully. (I am still not doing SD-card module initialization.)

Of course, I can just disconnect the SD-card module or leave the card in it... But I am trying to understand this black magic.

I have connected the SD-card module CS to pin 8, the CAN-Bus shield clone CS is connected to pin 10. I tried to set the SD-card module CS pin to OUTPUT and to HIGH. That had no effect.

the CAN-Bus shield + schematics: http://www.elecfreaks.com/estore/can-bus-shield.html [Resource -> Schematics]
the SD-card module: https://ebay.com/itm/161366316226?_trksid=p2057872.m2749.l2649
Without seeing schematics this is purely supposition:

My guess is that the SD shield contains bi-directional level shifting. The MISO pin, which connects the DO pin of the SD card to the MCU, will have a 3.3V -> 5V level translator on it.  This, with no SD card inserted, will be floating (flapping around in the breeze) causing the output of the level translator to randomly produce digital signals on the MISO pin. These signals interfere with the MISO signals from your CAN shield.

How best to cure the problem very much depends on what level shifting is in the circuit and how it is controlled and wired. For that we will need to see schematics of both the CAN and SD card shields.