The problems with manholes this time of year

They explode.

From the Times today: There have been at least 19 manhole fires or explosions since New Year's Day in New York City. Per the article:

All these incidents — only one of which involved an injury, minor — are part of an annual cycle as predictable as crocuses in March and mosquitoes in June: the surge in manhole fires as salty slush seeps down into the city’s maze of subterranean utility closets.

How to explain the manhole explosions then in other times of the year...? Such as in September ... or October...