Quetiapine (Seroquel) isn't technically a SSRI, it's a second generation atypical antipsychotic.

The "atypical" part is a reference to the fact that these relatively newer drugs originally developed to treat schizophrenia don't harbour as many as these terrible and often permanent muscle twitching side effects as their older counterparts like Haloperidol that would have been used in the One Flew Over The Cuckoo's Nest era that turned patients into paralyzed zombies.

Antipsychotic drugs developed to treat schizophrenia tend to target Dopamine receptors in the brain to try to block hallucinations as opposed to SSRIs in depression that target Serotonin.

This is why drugs like Quetiapine and it's predecessors have a unique side effect profile that mimics Parkinson's disease that similarly involves Dopamine receptors (tremors, paralysis, muscle rigidity, blank facial expressions, motor tics, trouble initiating muscle movements , etc).

Quetiapine was later used "off-label" at lower doses to treat non-psychotic diseases like depression and anxiety.

This mostly came about however out of desperation since the SSRI drugs are so shitty at actually treating the depression it was designed for.