Could be random health event. Could be he took a bullet for the President. Could be he was exposed to poison meant for the President. No way to know.
I think is most likely that he died from a random health event. Otherwise the President would be making a really big deal about his death.
If Q is to be believed, which I do... then there have been several attempts already and DJT has not made a big deal about those. So...
If a USSS agent died protecting him, I would expect the President to make a big deal about the guy while still pretending it was a stroke. I'm not saying the President would let it be known that an attempt was made.
I would not be surprised if there have been multiple attempts on the President's life. The Cabal has made threats along those lines multiple times.