My first exposure to Doctor Who, like most Americans of my age range, was the Fourth Doctor (Tom Baker), since he had the longest run of any actor on-air and was shown most often on PBS here in the US. But I’d say the Sixth and Seven incarnations probably made more of a lasting impression on me when I watched the show as a kid.

I don’t remember much of specific adventures of the Sixth Doctor, but I do recall him seeming to have a manic quality to him, especially soon after he regenerated. And it was from this impression of him that I thought a Velociraptor would fit him best.