Bumble bee piggy back bumble bee

Observation - Bumble bee piggy back bumble bee - UK and Ireland. Description: Large buymble bee - not sure which - but a smaller bumble bee sitting on its back and when disturbed the bee flew off carrying its pasenger. Was the smaller bee stealing pollen from the host perhaps? I have never seen this before.

Large buymble bee - not sure which - but a smaller bumble bee sitting on its back and when disturbed the bee flew off carrying its pasenger. Was the smaller bee stealing pollen from the host perhaps? I have never seen this before.