When a guy offers a fist bump?

We're friends for long and he never initiate any contact. The only time we come in contact is if we sit somewhere with friends, he sit close so our arms and legs touch (he is the one to arrange we sit next to each other) or if I nudge him with my body when he tease me (he never back away but stay firm and give "resistance" with his body). Now he offered a fist bump. Why now after so long into friendship?
He never initiate any contact with other people either.
When a guy offers a fist bump?
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