An additional component often added to the commonly used all over it
phrase, thus creating a simile. This helps to reinforce the point and also shows that the user of the phrase has a firm grasp of the english langauge.
When the standard phrase "Im all over that report" becomes dull..Instead it becomes "I'm all over it like a rash".
Shows the true passion and skill of the worker/user of the phrase or there could be some bull-shitting involved.