With heating, alkylboranes can isomerise to the most stable i.e. least sterically crowded isomer.

For example;

The reason this can occur is that the B-H addition becomes reversible at increased temperature, so the double bond can effectively move along the backbone until it reaches the end, at which point the alkylborane formed is the most stable;

This can be useful because if the final 'primary' alkyl chain is displaced from the boron by a less volatile one, the less thermodynamically stable terminal alkene can be collected - e.g.;