What an horrendous experience for you and I understand why you want to understand what happened.
I have never seen this and hope I never do. You mention that BonBon may have had brain involvement in whatever was her cause of illness and I wonder if that meant that her response was from a more primal part of the brain? Were that the case then she would not really have been aware of it. Humans can react what appears to be badly when they are moments from death and this is I think caused by only the brain stem still functioning and being the last bit to go. The reaction is totally automatic and only distresses the watchers
Or it could have been that because of the slower spread of the fluid, she felt it as it went into her vein - I think what they use is pretty powerful and unpleasant stuff but usually as you know it's all so quick that there is no reaction. You could ask to speak to your vet to discuss what he thinks caused the crying out. Explain how distressed you are and that you want to be able to face future similar occasions confident that there will be no repeat. Vets are no more fond of euthenasing animals than we are at having to have it done, so he was probably also upset, and should have revued for himself what went wrong, if anything was wrong.
Hopefully some of the Vets on TAS will express an opinion too.