Right now a lot of people are reporting having to answer a captcha due to "too many login attempts".
This happens now and then, and I haven't figured out exactly why. However, my current reasoning is that this is not due to a brute force against your account specifically. It is rather due to a peak in spam bot login attempts in general. Since there is a caching proxy in front of the forums, the forum software thinks all these login attempts come from the same IP adress (the address of the proxy) and thus starts showing a captcha for everyone who comes from that IP address (which is literally everyone since everyone goes through the proxy).
It's inconvenient, but I'm not keen on disabling the anti-spam measures we have.
Usually when this happens, it subsides on its own after a week or so.