Boba was with Jango when this happened... Personal experience...
Also, Boba probably knew that a ship that size couldn't have a cloaking device (see Captain Needa) so Solo was either A) still around or B ) went to hyperspace... If B then why earlier, unless your motor was damaged? So Fett figured A), and if the clamps on the ship couldn't take the loads in Hyperspace, he figured Solo would have to leave before the fleet went to Hyperspace... So floating away with the debris was a logical thought...