he was the chosen one because he likes to make stupid choices
he chose to try to stop megatron from pulling a fast one on prime,only to be yelled at first by prime,(out of the waaaaay hotrod),then used as a shield,causing the death of prime.
he chose to "dig" for the matrix,which was intended to be passed to primes "old friend".
by diving for the matrix and touching it first,the matrix realized it could have fun with a bot who kept making wrong choices