What was the explosion when Holden interrogates Leon?

The explosion is the gun going off -- in other words, muzzle flash -- and/or the beam (that purple light) subsequently coming up through the table.

From there, the beam passes through the thermos before striking Holden, which sends him through the wall.

To provide more info on the beam shot by the gun, here's an excerpt from a Blade Runner fan site that quotes Ridley Scott:

In the film, you can see the anti-matter blast that was animated into the shot for four frames. This effect was used only once in the film, even though Leon takes a second shot in that scene, only the first shot from under the table contains the animation of the anti-matter blast, this was due to the constant threats of going over budget and schedule.

"What I particualrly wanted to avoid in Blade Runner's weaponry was an indication of the common laser pistol." Scott said. "We all felt that a bright streak of light coming out of a barrel had become a horrible cliché, and we were sick to death of it. Then David Dryer came up with the idea that our pistols discharged a high intensity, particle-beam-type material that imploded on contact, drawing in so much light on the way that it became a black beam instead of a light streak. And whatever section of the body it hit would collapse in on itself. That would lead to a rather elegant demise - very little blood or gore.

"I thought this was an interesting concept" Scott concludes. "A black beam would have been the dramatic opposite of the expected laser ray. So we intended to later animate this dark effect and dub our weapons Black Hole Guns."