They’d observed a rash of burglaries in the area.
Trayvon had been suspended from school shortly before for “being in an unauthorized area”.
***TRANSLATION: TRAYVON IS A BURGLAR.***
That is what Zimmerman thought, but we don’t know that. They didn’t say why he was suspended. “Being in an authorized area” could mean ANYTHING at a school, most people might find the school to be overly sensitive and PC.
Maybe he took lunch hour early to eat with his girlfriend. Who knows?