Why was I forced to retire early?

What I Did

If I had been relying on autosaves, I would have been screwed and had to make a new character. Luckily, I am good about making manual saves, and had one from the playoffs. I loaded it, and simulated through each week of the offseason. For some reason, my current team did not offer me a contract extension. This seems a bit absurd, but maybe they couldn't afford me, or maybe there's some kind of bug related to re-signing, or maybe I missed it in the interface.

In week 3 of free agency, I received four contract offers. These showed up on the main To-Do tab. I couldn't find them in any other menus. I looked at which teams they were, then went to check their rosters. When I came back to the main tab, I found that the contract offers had disappeared.

I had to load again, and simulate back to free agency. I received the same offers, and was able to accept one. I've now played multiple seasons with the new team, with no further contract issues.

Conclusions

This is caused by simulating through the free agency period in a contract year.

When you create a new player for this mode, you are signed to a three-year contract. If you don't accept any new contract offers after your current contract expires, you automatically retire, with no way to undo it.

Once you click on the contract offers, you must accept one in order to avoid retiring. If you back out, the option will be gone forever.

I would recommend manually saving before entering the offseason, as this is easy to screw up.