Multiple factors are in play that affect error making. Cognitive load is an important element that increases the chance of making errors. Other factors are the presence of distraction, levels of stress or level of vigilance.
It's a matter of chance. In psychophysics, the field of research I'm spending most of my time in at the ...
In his 2003 psycholinguistics book, John Field has summarized (pp. 70-72) his own typing errors and combined them with an older corpus of Hotopf.
Missing words were among the frequent errors, but alas no numerical frequencies are given.
But what he says he noticed is that short function words like "are" or "it" (these are his examples) are more likely to ...