Grams didn’t die at the manor

Grams had a heart attack and fell from the stairs. She didn’t immediately die. She was brought to the hospital and later died there.

In the pilot, Piper told Phoebe that she met Jeremy there while she was crying. Jeremy knew then that once Grams died, the Charmed Ones’ powers would be reawakened. So he established a relationship with Piper and revealed himself when Phoebe returned.