Strange Behaviour?

We have blue tits nesting in a box in our garden, they have been taking beakfuls of nesting material for some time now but today as soon as one took a beakful in and flew off the other bird went in and came back out with a beakful which it then let go in the bushes - what do you think was happening?  Was the second birds pinching the nesting material or was it a pair who disagreed on where they should nest??

What is this world if full of care we have no time to stand and stare ...