Are they possibly using this as a reason to get me to sign it and
maybe sell the patent?
Usually, when we do this it's because paperwork got misplaced in an acquisition or otherwise. The per-patent assignment (at least for us) is a second layer of protection, as our employment agreement requires the employee to assign all relevant IP rights to the company. This is a standard clause that is in most employment agreements for any reasonably large company.
As you mention below, you may, in fact, have an ongoing requirement from your employment contract to assign that patent to the company. It would depend on the specific wording of the employment agreement and on the jurisdiction as to whether and how it would be enforceable.
I'd like to understand what it means if I do sign it. If they did sell
it, would I not get any royalties? Do I have to sign this assignment?
You're not going to be getting royalties. Unless you can resolve the ownership issue in your favor (unlikely) and want to spend a large amount of time and effort shopping the patent around to competitors, nobody's going to pay you.
Do you have to sign the assignment? Depends on your employment contract. I guess the worst that could happen if you refused to sign is that you're in breach of that contract. Are they going to take you to court because of that? Probably not.
They (the IP department) usually have alternative ways of resolving the issue (e.g. submitting the employment agreement instead of the assignment), if necessary. Practically, you're going to end up in the same place either way (they will own your stake in the patent), but not signing creates work and annoyance for their IP department.
I realize that I may have agreed to assign my patent rights, I haven't
actually looked at the text in the original contract What if they
can't provide my signed initial employment contract?
I've heard of a small payment being tendered in exchange for the signature when the company didn't have a copy of the inventor's employment agreement. The company still most likely would've prevailed in court, but a few hundred dollars was worth it to clear the cloud on the title without having to get the litigation team involved.