please explain mysterious monitor flickering to me

[kalpana]

Two computers, A and B, both running Ubuntu Jammy.

Monitor 1 and 2 are connected to computer A.

Via a physical switch, Monitor 1 can also be switched to be connected to computer B.

I start out with both monitors 1 and 2 connected to computer A. All is fine.

When I throw the switch to switch Monitor 1's connection from Computer A to Computer B, Monitor 1 works fine on Computer B. But, then Monitor 2, which is still connected to Computer A, starts flickering severely. Why does that happen? How do I prevent that from happening?