Well, for options it's more complicated than that - you can have ISOs that convert to NSOs to allow people to defer having to exercise illiquid options when they leave the company. So that's where my options question was coming from - the scenario you describe is VERY common in a scenario where ISOs have converted to NSOs upon leaving the company.
But okay, you have RSUs - how familiar are you with your agreement? It could have been a double trigger vesting arrangement, where the shares "semi-vest" over time, but then they don't fully vest until a liquidity event, at which point poof suddenly all of those ghost shares become REAL shares. If that's the case, they likely baked in a process for employees to have those shares withheld, or auto-sold during the lockup period. It's all tied in with their HR system and other payroll processes to make that easy.
Another scenario is that at some point since you left, or right before the IPO, they re-issued everyone's shares to be a different share class, because they wanted to clean up their cap table before going public. For employees they could just fix that for them, because again - all baked into the existing systems. For previous employees (and people who were gifted stock and former board members and advisors and angel investors and whoever else), they don't have an easy way to fix this. The old stock class technically doesn't exist because its been converted, so they can't sell to cover, and when they convert, they couldn't automate that because they don't have your withholding information and other payroll details for compliance purposes.
In either scenario, it's worth either reading your agreement carefully and/or talking to an attorney. Regardless, however, if this was related to an IPO, the legal and compliance stuff on this is going to be buttoned up and carefully done, so assume that (however unfair) they either have to do this in this fashion or it's much easier for them to do it this way (or some combination of both). It is possible to do very shady things during a private transaction, even a private transaction with a publicly traded company, but not for an IPO.
There are companies that will loan you money to cover vesting costs or these types of situations - they'll do it at shitty rates, but if the options are losing out on a windfall or losing an extra 10-20% on the windfall, it's worth considering.