Citalopram might be exacerbating them by a different mechanism. Serotonin isn't just a brain neurotransmitter. In fact this is only a minor function. It has many other roles in the body including in mediating blood vessel tone, constriction and dilation. Serotonergic antidepressants (ADs) can effect this so if the migraines occur often that could be the reason, though I suspect it won't be the only factor.
Excuse me adding to your information PDU : Though there is much evidence now, somewhat debated though I believe, that 'cortical spreading depression' (rather than any vascular changes) are the cause of silent migraine - so the blood vessel constriction and dilation are less relevant/not relevant for this type of migraine. Would that make the SSRI more or less likely the potential cause ? I agree with you though , that even if it was part of the trigger it isn't the only 'cause'. I restarted my SSRI only to tackle silent migraine and it has reduced them by half in total each month, and made them less intense - but they are not gone and there are clearly other triggers within my body that I'm not aware of.