We, who don't tow big trailers for a living, forget how much MASS a combination (tow vehicle plus camper) really is.
Newtons law of motion says: An object in motion tends to stay in motion unless acted upon by an external force.
Rolling down the highway we forget that the camper is back there. A sudden gust of unexpected wind; a passing tractor trailer; suddenly getting cut off; trying to pass and not giving the passed vehicle enough room; any number of unexpected events can result in an out of control situation from one heart beat to the next.
An out of balance load; incorrectly loaded camper (weight of camper vs pin (15-25%) or tongue weight 10-15%); length of camper vs tow vehicle wheel base; any or all of these can make a bad situation infinitely worse.
How can it happen?
Why it does not happen more often is a better question.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW