But in this case, the jet would have been following an instrument approach (probably RNAV 14) to Gaithersburg, which would have given him essentially a 10-mile long straight-in approach to the runway, with no need for this last minute turn. Also, Runway 14 has a "VASI," a set of red and white lights to give an incoming pilot a guide to the proper descent slope to follow. If you see all red lights, you're too low; all white, you're too high. Both red and white, you're on the right path.
7) To summarize #5 and #6: None of the usual weather-related, mechanical, or traffic-pattern problems that explain crashes seem to apply in this case.
Update 7A) The recording of radio traffic on the KGAI frequency contains several references to large number of birds around the runway. In my experience that's not rare, but it is conceivable that a bird-strike could have disoriented, distracted, or even disabled the pilot; or that maneuvering to avoid birds could have led to a loss of control; or that a bird going into an engine could have been the beginning of the plane's problems. The recording is here. [Update-update: see NTSB briefing below, which finds no evidence of bird strike or "bird ingestion."]
8) Gaithersburg is an "uncontrolled" airport, with no control tower. As the recent midair collision near Washington showed, control towers don't eliminate all traffic-conflict problems. But at Gaithersburg, pilots judge their position relative to one another through announcements on the CB-style common radio frequency. "Montgomery County traffic, Cirrus XXX is nine miles to the northwest, will make 45-degree entry to right-downwind for Runway 32." "Montgomery traffic, Cessna XXX is turning base to final for Runway 32." Etc.
9) Gaithersburg is very active as a training airport. On good-weather days (and yesterday was good enough to qualify) its environs usually contain a number of planes doing takeoffs and landings as part of their drill. Often they fly "closed traffic": taking off, flying the rectangular traffic pattern, landing, and doing it again. By definition, many of these are less-experienced pilots. A lot of them are non-native speakers of English, which means that it can take them longer to report their position and plans on the frequency, and sometimes to be less accurate about it.
10) The combination of points 8 & 9 can complicate the final stages of approach to landing at KGAI, in the following way: If you're coming in on an instrument approach, from ten miles out you're on a straight line for Runway 14. But the closer you get, the more you're alert for student (or other) pilots taking off, landing, or flying around in the pattern. I've kept count, and in recent months on about half the approaches I've made, I've had to make close-to-the-airport adjustments because of traffic in the pattern or whose location I wasn't 100 percent sure of. Sometimes this meant "going around," putting in power, climbing, and circling around for another landing attempt. Some times it means slowing down or making delaying maneuvers, usually "S-turns" to draw out the arrival process and sometimes full 360-degree turns. It's an expected rather than startling aspect of operations at this airport.