My theory is that The City was the primary force behind Brexit. They are by far the largest and most influential entity and one of the only few that are going to be better off after Brexit.
Even more, staying in the EU would actually jeopardize their special tax-heaven status as EU was moving toward more banking regulation. Note: EU rules or laws, once accepted by local parliaments, have the status of international law, which is above the local laws. Since The City has special exempt status in British legal system, accepting the EU banking regulation could potentially do away with their special "tax-heaven" status. So leaving the EU was actually a matter of survival for them.
Even more, staying in the EU would actually jeopardize their special tax-heaven status as EU was moving toward more banking regulation. Note: EU rules or laws, once accepted by local parliaments, have the status of international law, which is above the local laws. Since The City has special exempt status in British legal system, accepting the EU banking regulation could potentially do away with their special "tax-heaven" status. So leaving the EU was actually a matter of survival for them.