Antipsychotics really helped with going from mixed to textbook rapid cycling, for me. Atypicals keep a balance. Any meds working just for depression may jeopardise that balance. You can get sudden, strong anxiety by lack of anxiety.

Then (very short) mania will decrease that anxiety and you might get strong anxiety again, causing (short) depression, which decreases the anxiety, being further reduced by the med, causing strong, sudden anxiety from lack of anxiety. Could be that. Hypervigilance. Growing fear for mood fluctuations.

For me, panic attacks help: they stop the cycling for some time. So maybe a little or very much anxiety is best for mixed. And more stable perception due to antipsychotics.