New words are often created through mistakes. The most common of these errors is something called back formation or false analogy. This means that someone sees an existing word, falsely places it in a particular grammatical category, and then forms a new word based on that false assumption. For example, the noun pease originally meant what we know of today as the pea. However, the word pease was both singular and plural (something like pants or scissors). But sometime around 1600 someone thought pease was plural, so they started using the word pea as the singular.
Note: The Oxford English Dictionary lists over 700 such back-formations, including agoraphobe from agoraphobia, exurb from exurban, sleaze from sleazy, burgle from burglar, enthuse from enthusiasm, televise from television, couth from uncouth, ept from inept, and gruntled from disgruntled. Other examples are surveil, baby-sit, diagnose, donate, and reminisce.