I was reading over the paper and found the source of that particular usage:

Furthermore, in a study by Henneberg and George (1995; Am J Phys Anthropol 96, 329–334), has suggested that increasing prevalence of the median artery during the 20th century was a ‘possible secular trend’.

LOL. I kinda want to follow that citation for the full quote.

Edit: I found the original source that gives some further context:

The occurrence in historical times of changes in human body size and in the timing of events in postnatal development, such as, for instance, sexual maturation, is well known and documented. Such changes occurring from century to century or decade to decade are known as “secular trends.”

So I guess it’s actually domain specific jargon.