A saeculum is a length of time roughly equal to the potential lifetime of a person or the equivalent of the complete renewal of a human population. The word has evolved within Romance languages (and Swedish) to mean "century".
Doesn't that come from centum (100) like cent and centurion?
Of course it's MBA-speak. MBA-speak does not mean that they invented the word, or that only MBAs use the word. It just means that MBAs prefer to use the word where another one would do.
Nadella could have talked about "long-term trends" or "ongoing trends" or "sustainable trends." However, he chose to use "secular trends." His fondness for jargon makes him harder to understand.
Another example of MBA-speak is "synergy." The OED traces "joint action, cooperation" to 1632, and "a combined effect which is greater than additive" to 1904 ("synergism"). The first listed MBA-speak usage of the term is a 1981 article in The Economist on brokerage mergers.
Does that mean that "synergy" is "not MBA-speak" because the term is also used by scientists? Of course not! It just means that the MBAs adopted a term that others had already been using.
I would place "secular" into the same category as "synergy." The OED traces its astronomical usage to 1801 and its economics usage to 1895. Its first appearance outside a scientific context is in 1973, in an article in The Daily Telegraph on interest rates.
It's not "MBA-speak"; this sense of the word "secular" is used in economics, time-series analysis, astronomy, etc.
https://en.wikipedia.org/wiki/Secular_variation