According to NHTSA study, increasing load weight
dramatically increases risk of roll over.

For example, on 15 pass. vans,

moderate load = 13% of accidents = rollovers
full load    =  36% of accidents = rollovers
overlaod      = 70% of accidents = rollovers

http://news.excite.com/news/ap/010409/17/news-vans-rollovers