I made the jump because more and more of the products I was working on had some sort of interface, and that’s what I tended to focus on improving on. In a small consultancy it didn’t take long to become the go-to guy.
I do think that Industrial Designers tend to make the best Interaction Designers because they tend to ignore discipline boundaries and focus on the end-game of making the product experience better for people (making the world a better place et. al.)
I’ve found that those practicing Interaction Design from the traditional routes of academic HCI or Software Engineering tend to be more focused on building the thing itself, or focusing on obscure sub-specialties in order to publish scientific papers.