I think what TIM was trying to say in his 2nd point was that the physical jarring caused a hiccup in the software's I/O procedures. Think of it like this: you're writing a file that takes up sectors 00 to 04. However, the laptop is jarred, things get a tad scrambled for a second, and while the file had written the first two sectors (00 and 01), the jarring caused it to skip a sector, and afterwards, wrote to sectors 03 to 05. Well, it doesn't know that it wrote to that last sector, 05, it assumed that the writing was successful, but it has in fact, corrupted not one, but two files in the process.
That, and a drop could cause the reading and/or writing heads in the physical HDD itself to skip, or perhaps even dig into the HDD platters themselves. Having taken apart hard drives in the past, I can attest to how sensitive the platters are in non-solid state drives; they're basically aluminium discs coated in...brass or something, some sort of bronze coloured metal, and they function more or less like old-school record players.
Okay, done rambling, lol.