Can you be more specific? What were the FMAs? Was your cruise speed slower than your decent speed? Did you descend before, at or after TOD? Did you speed intervene? Where was the decent path? Did the FMC recalculate? Did you add any constraints that might have forced a geometric path?
In HDG SEL it should still cycle waypoints and calculate the path based on the distance to the next waypoint so it could be that your use of HDG SEL was entirely coincidental.