"It's a trap!"
Admiral Ackbar, Return of the Jedi
Most people know that bad-mouthing a previous employer tells an interviewer more about the candidate than it does about the employer, so they've spun it so it looks like a positive question. It will be taken (by the interviewer) in exactly the same way as an answer to "Tell me what your previous employer did wrong".
It buys into the idea that people leave a company to get away from a company. True, some people do that, but those are generally the ones who are less successful in interviews : if this candidate is willing to criticize their previous employer, what are they going to find to criticize us?
You could reflect it back at them. "Nothing I can think of. They were a great company and I loved my time there. I suppose I'd say "Just keep doing what you're doing"." [Though don't say all of that if it's not true - stop after "Nothing I can think of."]. If what the interviewer really wanted to know is why you left, that'll prompt them to ask that directly. This is your opportunity to talk about the positives in the company where you're currently interviewing. "They were great. But you're better."
People change jobs for better opportunities, interesting work, more pay and different responsibilities. The ones who emphasise the positives at the new place will come across better in an interview than those who consider the old place stagnant, boring, cheap and stressful.