Ice is definitely a fickle thing. You may have come through a lot of visible moisture on a flight with nothing but a light dusting, then, as you come in from the outer marker, whammo--it builds up as if it's trying to convince you there's no tomorrow.
Comment was made herein that someone was surprised that a chuck of ice wouldn't break up in the slipstream, as it was not "aerodynamic." Though I'm surprised how far down the ET ice was forming (or perhaps traveling--channeled down grooves?), one has to note that roughly half the shape of ice would be as a result of a structure that was designed to be at least somewhat aerodynamic, such that, if it presents itself to the relative wind and behaves as something other than an irregular mass in low-velocity turbulence, it should surprise no one.
Fickle as ice seems to be, it forms in areas where moisture is generally visible. That is, when the air temp meets or falls below the dew point. If the visible moisture is ice, it usually will be resistant to sticking (melting sufficiently then refreezing), but not always. So-called supercooled droplets which have some, presumably environmental reason for not crystalizing (impurity, micro-turbulence, ?), may all-too-readily "decide" they have an affinity for some passing by structure (whose surface temperature also tends to be below freezing).
In my limited(thankfully) first-hand experience with such phenomena, those structures which pose the greatest divergent angle to the relative wind are most prone to build-up, apparently because water molecules "linger longer" and can contribute to uncohesive airflow (turbulence). This is to say that ice seems to form most readily on those wetted areas which directly impact the relative wind. Ice formation farther removed from airflow diversion is unusual in my understanding.
Supposing it's ice I saw in the clips under discussion, it seemd to me the ice departed the ET fairly near to the shuttle's nose. That's unexpected! I wonder if the surface air pressure on the ET in such a vicinity is reduced by the aeodynamics of the shuttle itself. Could the shuttle's aerodynamics "help lift" ice from the ET body, through what must be a really high-speed, largely cohesive relative wind. The camera angles I've seen (and what little I could discern) tempt me to conjecture that the "ice" came toward the shuttle at its steepest angle immediately after leaving the ET (though my eyes could have been fooled), then the angle thereof decreased as it neared the shuttle body, only to impact near the port leading edge. To maintain such a divergent angle across a Mach One relative wind bespeaks one astoundingly large mass "launched" in that direction. Did it slide off some structure as if it were a ski jumper? If those suppositions are at least partly true, it could be that the configuration of the two main bodies and aerodynamics in those areas actually increase the possibility of being struck by ice, should ice be present. (note to self: check correlation of increased problems with winter weather.)
WRT channeling of ice along the surface of the ET, I'm led to wonder whether differential surface temps on the ET contribute to an uneven ice buildup or detachment likelihood.
Because of the latitude and proximity to moisture-bearing weather, potentially ice-laden Florida clouds are taller in the atmosphere than they would be elsewhere, as the troposphere goes to its greatest heights nearer the equator. Thus, a shuttle's lingering through such potentially ice-laden environment is greater than elsewhere, and such circumstances may be outside the ken of those trained in higher-latitude environments.
Translation: don't launch that the shuttle in the winter when clouds of any significance lie in its flight path.
HF