I agree that the bridge is a nonsensical place to try and dispose of human remains.
I think that the taxi driver will be the key to a lot of information as to where this man and his suitcases came from, where he was wanting to go, and probably a lot more contextual information about how he behaved, whether he seemed to know the area, etc.
I suspect something alerted the taxi driver's suspicions or made him feel unsafe, which made him go to the bridge knowing that it had CCTV and would also be a busier environment, rather than, say, to nearby Leigh Woods, which is a more logical place to try and dump the cases and may have been the original destination. He may even have told the man to get out and left him to it, knowing he'd struggle to manage the cases alone and likely be stopped in possession of them. Pretty sensible if so and may even have saved his own life.
jmoo