Van der Waals strain, or steric strain, occurs when atoms are forced to get closer than their Van der Waals radii allow.  Specifically, Van der Waals strain is considered a form of strain where the interacting atoms are at least four bonds away from each other. The amount on steric strain in similar molecules is dependent on the size of the interacting groups; bulky tert-butyl groups take up much more space than methyl groups and often experience greater steric interactions.
The effects of steric strain in the reaction of trialkylamines and trimethylboron were studied by Nobel laureate Herbert C. Brown et al.  They found that as the size of the alkyl groups on the amine were increased, the equilibrium constant decreased as well.  The shift in equilibrium was attributed to steric strain between the alkyl groups of the amine and the methyl groups on boron.