The expansion of trapped moisture can result in internal separation (delamination) of the plastic from the die or lead-frame, wire bond damage, die damage, and internal cracks.
This occurs when part temperature rises rapidly to a high maximum during the soldering (assembly) process.
This does not occur when part temperature rises slowly and to a low maximum during a baking (preheating) process.
[1] The times indicate how long components can be outside of dry storage before they have to be baked to remove any absorbed moisture.
MSL-specified parts must be baked before assembly if their exposure has exceeded the rating.