Why are poblanos sometimes called pasillas?
I have not been able to find any kind of detailed history of the pepper crop in California online.

However, I will observe that I've only seen chilaca peppers available in Northern California in the last 5 years.  Previously, one never saw them, even at Latin markets.  On the other hand, California grows enormous quantities of poblano peppers, which are larger, hardier, and have a longer shelf life than chilacas when fresh.  Based on that, I suspect that some enterprising, dishonest produce supplier started supplying ancho peppers as pasillas to Mexicans in California decades ago, and after several decades of misnaming, the new names stuck.

I'll also observe that the dried poblanos sold as "anchos" in NorCal are a bit hotter than the ones sold as "pasilla", so it's also likely that there's two different varietals, bred here.  So it's also possible that whoever bred the milder poblanos named them "pasillas" to differentiate them from the hotter ones, confusing everyone.