H.W.
Oh, yes, dump and sort is quite the activity of the younger set! Susan got it right-- it's about the development of sorting and categorization skills, and kids need loads of hands-on practice to do this. Today, my preschoolers were playing stone soup in an old dutch oven and it had everything in there: 4kinds of nuts, pretend pasta, wooden veggies and fruit, and the stone.... and we took about 5 minutes sorting it back out, using tongs and fingers and having fun "making the little kitchen beautiful" again.
Toddlers do a 'dump and load' sort of thing, dumping baskets out. My almost 4 yo son likes to take three or four collections (puzzles, lacing beads, sorting spindles, whatever madness he can find) and dump them together. I do have to sit with him and help him sort them out, or give some clear directions, as it can be fun to dump but he'll need support for sorting. (This can be overwhelming for little ones.)
I also wonder, if your son is watching Thomas, I know there are quite a few of those stories where the trains have coal or snow or other objects dumped upon them and they get stuck. So he might also be incorporating that storyline into his play. But don't worry-- totally developmentally normal. I always just make sure that when these big "dumps" happen, that we get things put away before we get out more toys.