Air carrier Captain reported the aircraft rolled 5-10 feet after pushback and tug disconnect; citing distractions as contributing.

Narrative

We had just completed engine start and the tug crew had disconnected the tug and verified its release. After releasing the tug crew I realized the aircraft was beginning to roll. I looked and realized I had not set the parking brake. I estimate the plane had rolled 5-10 feet. I immediately stopped the airplane and set the brake. I attempted to contact the tug crew on intercom but they had disconnected and were pulling away. I do not believe fatigue was the primary factor. We had been delayed on the ground based on an ATC ground stop from Center for 25 minutes. That through us a little out of our normal rhythm. I do not recall if the tug crew asked me to set the parking brake after completing the push pack. We were in the middle of an engine start and there were some other ground communications taking place as well. I asked the First Officer if he remembered hearing the tug crew ask me to set the brake: he did not remember either. They may very well have done it: I just normally recall the communication well on the parking brake pre- and post-start.In almost 6 years as a Captain this is the first time this has happened to me. I should have verified as I was releasing the tug crew that the parking brake was set. That is my responsibility. I will brief in the future the danger of distractions and the importance of taking more time when we are thrown out of our normal rhythm.