Why is Y!A all of the sudden popping up with "I'M not a robot" things?

The assumption is that it's been done to reduce the bot accounts that frequently post on the site, and to prevent more spam bot attacks like the ones last year that flooded the site.

It would require extra programming to ensure your answers were not 'irrelevant' to the question asked, so it's easier to hire another company (in this case Google) to supply the captcha program for security, as it's unlikely a bot program could provide the correct responses (especially to the visual image format being used).

A lot of users are starting to see it its likely a New Spam Filter that Yahoo is Testing

Its Annoying yes but nothing you can do but go through It

if its a new system to try to prevent bots from answering, it's clearly not working. i've also seen a couple of them.

It is an anti-spam measure.

to prevent spambots like the one here