That’s awesome. If I ever get sentenced to life in prison, I’m going to demand 24/7 internet access and read that sucker. (Provided I live long enough.)
From reading most of the available books published about the SR-71 I’ve never encountered any suggestion that the films were deployed like this. I don’t have the literature to hand (I should at the weekend) but I recall the film reels were rather large and quite sensitive so I can’t believe they’d design them to be ejected. There would also be a number of reels of film, for each of the different cameras loaded, so if this was a “safety/urgency” ejection, I’d expect more than one cannister. Plus, the sensor pods were in the belly of the plane so there would have to be a system to move the film to the upper rear of the plane, through the complex engineering, fuel cells and magical gubbins that made it such an awesome plane.
They also carried radiological and chemical sensor/capture pods so I’d expect that if a “can’t wait for the plane to taxi” ejection system was built for the film, they’d also provide a similar mechanism for the sensor pods.
A WAG is that what you saw was possibly part of the braking 'chute system; possibly a cover that popped off to release the drogue chute which would then extend and flare the main 'chutes.
The books also stated that the support truck was driven by fellow pilots and had radio comms to provide any guidance required by the 'bird’s pilot.
One of the better books I read was by a “Habu” pilot who spent some of his time at the base you described. Reading about how they’d maintain flight control at cruising speed by turning small wheels to make minute adjustment to the plane’s control surfaces and inlet spikes was very enlightening. I think I’d previously assumed they just flew stick at mach 3 
There were also a number of anecdotes about how hot the plane would be after landing and how they’d have to wait before opening some of the access panels or getting a maintenance crew onto the plane for turnaround.
If anybody is sufficiently interested I’ll try to find the books when I’m home at the weekend.
t.
Yes, please do.