The city names are just the locations in which those treaties were negotiated. The negotiations over the Maastricht treaty were finalised at a meeting of Heads of Government in Maastricht.
Treaties are multilateral agreements between governments. When the various governments of the EU come to a mutual decision that there needs to be some change in the agreements that bind them together, they will begin the process of negotiation. The initiative comes from the European Council, and the detailed drawing-up is done with the Commision.
When a draft agreement is ready, the European Council will agree to meet in a European City, usually a city in the country that is holding the rotating presidency of the Council. At that meeting, the heads of government will try to agree a final text for the new treaty, and if successful, the new treaty is named after the host city.
So new treaties are drawn up as-and-when required by the European Council and it is the Council that initiates treaty change.