I am a little surprised how he managed to run away carrying the heavy back seat with him while the guard who might have been chasing him, couldn't out-run him.

Reasons why he chose to steal a car's back seat may be that he was a mechanic in a garage earlier and he knew how easily he could do this job and sell it at the right place. It's easy for a car mechanic to steal car parts or a software expert to hack into computers...it depends what's your acumen.