One might certainly wonder why somebody would think that was a good way to fire somebody, but it seems to be something that gets done fairly often.
That type of termination is usually for a breach of company policy (e.g. flagrant expense account abuse, harrassment or some other lapse of integrity). Perfomance based terminations are usually much less abrupt. Seen both types - neither is pleasant for the victims or survivors.
Yeah, my other thought was some kind of intellectual property violation, where you wouldn't want the offender to have a chance to grab any more information or destroy evidence.