Fear of citizens burning down city hall? It's usually difficult enough to pass ordinary tax increases. Trying something outrageously unpopular like that would probably be political suicide.

What politicians do is just narrowly target a proposed tax to apply to a small group of people that they can paint in an unsympathetic light.

So rephrasing the question, "what exactly stops a city from enacting a 100 year retroactive income tax on those in the 1%?"

Those with the most to lose are the least likely to burn down city hall.

"what exactly stops a city from enacting a 100 year retroactive income tax on those in the 1%?"

They will have no funds for their re-election campaign. In an extreme case they may die of a mysterious stroke or have an affair suddenly made public.

What if they just pass the tax retroactively on those that are the 1% that moved out of the city and are no longer involved in funding the politics?

The idea that something can be passed retroactively in any form is insane.

What stops a city from enacting a non-retroactive tax of equivalent value?

Not getting Re-elected. But if you tax dead people who don’t live there anymore that’s not a concern.