To answer your original question (I realize that it was essentially a rhetorical one), the #1 reason for getting into an unplanned deco situation is diver inattention. There are certainly other factors which could cause it: dive computer malfunction, incorrect nitrox setting, incorrect personal adjustment/altitude setting, helping effect a rescue, etc. In all of these cases (except for computer malfunction), the diver should still be "aware" (or at least anticipating) that he's exceeded NDLs.
Newer divers can get really flustered if their computers start beeping/flashing at depth. It's a combination of not being familiar with the deco mode of the computer in question and perhaps a touch of narcosis. I've witnessed more than one diver who freaked out a little when this happened. It can lead to a dangerously rapid ascent if the diver is not responding in a calm, appropriate manner.
One thing that some divers don't appreciate is that two divers can be diving an identical profile, i.e. have the "same" nitrogen loading, and be on either side of the "deco line." Or to put it another way, the dive profile can be such that the diver using the more conservative computer enters "deco" mode whereas the the diver with the more liberal computer could still be well within the "yellow" nitrogen-loading zone with double-digit NDLs available. Let's say both divers ascend under control, complete a 3 min. stop at 15 fsw...and then surface. The diver with the more conservative computer is in "deco violation" mode flashing arrows to the diver to descend below the ceiling immediately. In contrast, the diver with the more liberal computer is happily calculating surface interval time. In this example, which diver is exposed to the greater DCS risk?