Consensus cause of recent sargassum invasions is that increased discharge of organic matter and nutrients from rivers, mainly the Amazon, as a result of defosteration has led to formation of a new 'Sargasso Sea' off Brazil - its tail extends to the West Indies. Warmer water temps have been conducive to growth.
Hoped to be a seasonal anomaly a few years back, sargassum invasions of the Caribbean are expected to remain regular occurrences.
I agree with Amy . . . from day to day, very difficult to predict where you’ll find it, & the Collectivié is very good about jumping on clean-up of stuff that washes ashore.