Recently, I have noticed that it seems to be a relatively recent trend to only capitalize the first word (and names) in the title of a journal article. Some examples from the 21st century include:
Higher composition laws I: A new view on Gauss composition, and quadratic generalizations
The density of rational points on curves and surfaces
Prime values of a sparse polynomial sequence
This is contrasted with older articles such as:
Rational Isogenies of Prime Degree
In fact, in the last example, the "new" convention is applied to the webpage of the article, but one clearly sees that the pdf file, obtained by scanning the original article in the print version, follows the "old" convention. This suggests that at some point there was a deliberate effort to transition to the "new" convention of only capitalizing the first word of each sentence and proper nouns.
Does anyone have an insight as to when and why this happened? Perhaps this only happened in some specific subject areas or journals. However, the fact that a very general journal such as Inventiones seems to be doing this deliberately seems to suggest that this applies to all of mathematics.