[1] Such words usually have a prefix or suffix that would imply that there is an antonym, with the prefix or suffix being absent or opposite.
[2] Unpaired words can be the result of one of the words falling out of popular usage, or can be created when only one word of a pair is borrowed from another language, in either case yielding an accidental gap, specifically a morphological gap.
Other unpaired words were never part of a pair; their starting or ending phonemes, by accident, happen to match those of an existing morpheme, leading to a reinterpretation.
The classification of a word as "unpaired" can be problematic, as a word thought to be unattested might reappear in real-world usage or be created, for example, through humorous back-formation.
In some cases a paired word does exist, but is quite rare or archaic (no longer in general use).