Your not alone. My pond has done this every year since it was renovated 4 years ago with the first year being the most explosive green. I still get very heavy blooms, but they tend to be much more brown now. I have no good explanation besides nutrient loading from the surrounding cattle fields. ~90% of my watershed is cattle pasture that gets fertilized once a year and, of course, cattle manure. It has not caused any problems that I have noticed, besides being rather unsightly.
Here's my old thread the first time it happened...
https://forums.pondboss.com/ubbthreads.php?ubb=showflat&Main=35937&Number=470475#Post470475
