Possibly too busy reading his pre-flight checklist to properly pre flight the aircraft. Also check tie downs untied may not have been on the list
If only 2POB, 20Kg on each wing shouldn't push W&B outside the envelope. Prolly a bit more power required to taxi but I suspect not a lot - once the static friction's lost there wont be a lot of dynamic friction from a concrete block on tarmac.
I would imagine the pilot's feeling pretty bad about it, and is no doubt in for loads of abuse on the various forums. But let those who are without sin cast the first stone (or concrete block even
) I for one know that not all my preflights are perfect, I once took off with the pitot cover still attached
