What is the "chalk talk" at job interviews?
I can't comment on how common this is outside of biomedical sciences (my field), but it's becoming quite common in this field. 

In terms of what it is, the "Chalk Talk" is a chance for you to talk about your research (past, current, and future directions) without hiding behind Powerpoint slides.

You'll typically be given up to an hour to do this, in front of the hiring committee and/or faculty. You'll have a whiteboard/blackboard/flip-chart so you can draw down a key figure or two, or write an outline of what you're speaking about. It's less formal than a normal talk at a job interview - if you are asked to do one, expect to be interrupted and questioned throughout. 

There are many reasons these are becoming popular. It's easy for candidates to talk through a deck of slides they've had weeks to prepare, and the hiring committee doesn't necessarily learn that much about them as people or scientists. This is more dynamic - it lets you see how people think on their feet - and gives a real opportunity for people to display their passion. It's daunting to speak for an hour with no props or prompts, but a good academic who knows their field, has a clear research plan, and cares about their work should be able to do this.

Source - personal experience (I have been on training courses specifically about giving chalk talks in faculty interviews, albeit in a different field).