It seems very common (particularly among non-native English speakers, but I've seen the typed form, though not the spoken form, from native speakers too) for people to type or even say "substraction" when they mean "subtraction".
What causes this error? There must be some loose rule that the word "subtract" fails to follow, to cause the -s- to be inserted so commonly. Is it a phonotactics thing?
I'm not sure if this is the best place to ask this; it's possible that english.SE would be a better place for this. Feel free to move it if so.
The combination bt doesn't appear very often in English (often where it appeared in Latin, it's been simplified in English pronunciation, like "subtle"), while bst appears in words like "abstract". I would chalk this up to analogy with the relatively common and similar-sounding word "abstraction". (In the comments, jogloran also mentions "distraction".)
I've noticed this too (particularly in non-native but high-level English speakers), but I'm not sure this question is answerable with firm evidence. Different speakers may make the error for different reasons.
If I had to guess, it could be interference from the words "abstraction", "distraction", but note that Spanish actually has sustracción, sustraer, etc, and French has soustraction /sustʁaksjɔ̃/, which might lead speakers of such languages to analogise to *substraction in English.
According to Wiktionary.org, the usage of "substraction" is common amongst non-native English speakers due to the similarity to the French "soustraction" and the Spanish "sustracción", which both derive from the Late Latin "substractus".
I'm not a linguist but I have (unscientifically) noticed as a native American English speaker that many people mispronounce words.
Some words are graced with this more often, with the mispronunciation sometimes even making it into an official source like a dictionary.
Sometimes letters are added, sometimes they're rearranged and sometimes they're removed. The s-sound and letter s seem to be frequent problems, such as with ask (as axe), espresso (as expresso), and even the recent persistent mis-pluralization of words like mom (as moms).
Consider that through the course of a primary education, most native American English speakers are introduced to subtraction (or substraction, as you point out) because it is a core concept in elementary mathematics. They may have cause to use the word often but some combination of society misleading them, difficulty saying certain letter combinations, underdeveloped speaking ability, various speech pathologies, and laziness may cause them to say it incorrectly. Once learned, it may never be corrected.