Why did my cat go septic?

Today I had to put down my 5 year old cat, very unexpectedly. It started with vomit and gagging. We took him to the ER where he got bloodwork done and some IV fluids. We went to another ER for an ultrasound where they saw his intestines were filled with fluid, which they tested and revealed to be septic. He was in critical condition and fading fast. We were quoted $12-$17k for abdominal surgery and little to no odds of survival. We opted to euthanize him and the vet offered to do her own necropsy to see what happened. She called me and told me his stomach, bladder, and colon all had bruising and he potentially had a blood clot which caused this. She did not see any perforations which would’ve caused sepsis.

Has anyone seen this before? Does this all sound correct? I’m not doubting her skills, but with his critical state I really didn’t have a choice in getting a second opinion.

I’m just grieving and stuck wondering what happened and how it happened so fast.