B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Type 574
Situational Awareness
Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Incursion Runway
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Human Factors
Approach Control slowed us to final approach speed a couple miles outside FAF; so we configured fully and slowed to 135 knots. We were following a Regional Jet and had them and the runway in sight. Heard another flight check in behind us on the approach. We landed uneventfully on Runway XXR knowing the other aircraft behind us must be close; so I took the first available right turn; [onto] Runway YY. ATC did not tell us to expedite off the runway; but I knew the traffic must be close behind us as we had been doing 135 knots for the last six to seven miles of our approach. Tower Controller saw us turning onto Runway YY and directed us to return to Runway XXR; which I began to do and could have completed safely. Tower then revised the instruction to remain on Runway YY and exit at Taxiway A and contact Ground; which we did. We were informed that another aircraft had been issued a take-off clearance on Runway YY prior to our entering that runway. We were both surprised to learn of that. ATC then rescinded the takeoff clearance of the other aircraft. We completed the taxi to the gate uneventfully. I contacted ATC via the phone number they had given us and provided my name and certificate number per their request.Upon completing the flight; the First Officer and I read the AIM again; specifically part 4-3-20; which addresses our error in departing an active runway onto another runway. I genuinely believe that would not have occurred had I not been concerned about the aircraft behind us on the approach. I was also surprised to learn that another aircraft on an intersecting runway had been issued a takeoff clearance prior to our passing through that intersecting runway. And; of course; I was concerned that we had created a conflict. I don't know the Controller's responsibilities in that regard and of course I have operated many times when local operations were on intersecting runways; but I do not recollect that happening before. Regardless; we certainly did not intentionally create the conflict and I truly regret my error.
[Report narrative contained no additional information.]
B737-700 flight crew reported turning onto an active runway after landing due to concern that traffic close behind.