In practice, not all rule violations will result in a disqualification as the referee may use their own judgement and is not obligated to stop the match. Usually, the only offenses that the referee will see and immediately disqualify the match on (as opposed to having multiple offenses) are low blows, weapon usage, interference, or assaulting the referee. In WWE, a referee must see the violation with their own eyes to rule that the match end in a disqualification (simply watching the video tape is not usually enough) and the referee's ruling is almost always final, although Dusty finishes (named after, and made famous by, Dusty Rhodes) will often result in the referee's decision being overturned. It is not uncommon for the referees themselves to get knocked out during a match, which is commonly referred to by the term "ref bump". While the referee remains "unconscious", wrestlers are free to violate rules until the referee is revived or replaced. In some cases, a referee might disqualify a person under the presumption that it was that wrestler who knocked them out; most referee knockouts are arranged to allow a wrestler, usually a heel, to gain an advantage. For example, a wrestler may get whipped into a referee at a slower speed, knocking the ref down for short amount of time; during that interim period, one wrestler may pin their opponent for a three-count and would have won the match but for the referee being down (sometimes, another referee will sprint to the ring from backstage to attempt to make the count, but by then, the other wrestler has had enough time to kick-out on their own accord).
What are the usual reasons for an automatic disqualification? 
low blows, weapon usage, interference, or assaulting the referee