In studies of Brazilian toponymy, one of the procedures most used by researchers in the analysis of geographical names is the application of a taxonomic model. Dick's (1990b) model, which aims to classify motivating elements objectively and synchronically, includes, for example, the category of chrononotoponyms: toponyms with chronological indicators represented by the adjectives new and old. When highlighting the specific causes, which involve the reasons for the denominator, it is necessary to find out at least some aspects of the history of the named geographical feature; that is, the perspective becomes diachronic. The objective of this study is to demonstrate that in a category like the one analyzed here, the denominative causes, although specific for each designative, can be systematized and divided into groups according to their semantic content. For this, the list of names of Brazilian municipalities, published by IBGE, was consulted and 137 chronotoponyms were selected. Then, aspects of the history of each municipality were investigated (especially from IBGE's Cidades portal and the official websites of each municipality). With regard to theoretical reflections, the study draws on Lognon (1920), Dauzat (1947) and Dick (1990a, 1990b, 1999). The main denominational causes of chronotoponyms were found to be: a) when a human settlement which has already been officially named is promoted to the category of municipality, it adds the adjective novo/a (new) to mark a new time in the history of its development; b) when a municipality is named after another city, state, country, etc., it includes the adjective novo/a (new) to avoid homonymity. In addition to these, other causes are described.