Rod bearings gets enormous loads of stress and top chart on the list of catastrophic engine failure. Tight or stiff bearing issues can be the result of many reasons that lead to their malfunction. Rod bearings can spin, freeze, stretch, deform and suffer out-of-round conditions. Rod bearings have precision, machined surfaces that must operate in a contamination-free environment during inspection and repair. However human error, lack of lubrication and some structural failure account for most bearing crisis. Foremost among it is HEAT
When inner rod bearings have reduced gap between the bearing surface and the crankshaft journal, a correct oil film has no room to enter between the two surfaces. This increased friction causes heat, causing the bearing to expand in the race, further sealing off the oil delivery. When temperatures increases as high as 400 degrees, it results in annealing or galling of the bearing to the journal race. This will often tighten or freeze the bearing against the crankshaft journal. Blue-black marks will be seen on bearings and journals that have suffered extreme overheating.
http://www.automobileglobe.com/effects-of-heat-on-connecting-rod-bearing/