I just did a quick search, and found this:

Concurrent use of stimulants with antidepressants is a high-risk behavior and is medically contraindicated due to potential cytochrome P450 2D6 (CYP2D6) enzyme inhibition and a resulting increase in serotonin levels.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4309786/

So it was the common culprits, the P450 enzymes. I’m lucky to have not given myself serotonin syndrome many times.