It isn't that surprising, LOR requests are all about timing and yours was just good. This time of the semester is a lot less hectic than it will be in a few weeks, so they just want to get it done and not forget you. Good letters of rec are easier to write than borderline ones too. So, I think you can relax about this and find something else to worry about.
Asking profs for letters during less peak times, like during breaks or early in the semester is a good idea. It shows respect for their schedule.