The formation of middleweight black holes with masses between 100 and 10,000 suns is rather speculative and the information available is sketchy, mainly because of its recent discovery (1999). 

    It is believed that middleweight black holes form from the combining or coalescing of many smaller black holes. When two black holes coalesce, their masses are combined to form a larger black hole.

    In order for such a high rate of collision to occur, these black holes most likely form in irregular galaxies with high rates of star formations such as M82.