Trinkets of some value kept going missing from around the house. Alice was sure that her daughter was secreting them out into the gardens, then stashing them in some hidey hole. When a gold necklace, complete with diamond pendant, disappeared, Alice was sure that the elopement was imminent, though her daughter hotly denied it.
Then Eliza herself could not be found around the house. While servants searched, the anxious Alice went rushing out into the grounds, yelling her daughter's name and looking for any lantern light. Sure that she had seen just that out by the old well, she raced in that direction. But this was night-time and the boggy ground was slippy.
In some tellings of the story, Alice tripped and banged her head, dying almost instantly. In others, it's much more dramatic and she ended up in the well, drowning before anyone heard her screams for help.
Either way, she died that night, but that was not the end of her search. Not by a long shot. It's said that if you're ever in those woods, up the mossy bank behind the manor house, you may encounter the distraught Alice. She's not done yet.
(And incidentally, genealogical records show that Alice was either mistaken or paranoid to the point of being deranged; either that or guilt got the better of Eliza after the accident. The woman married a local squire; a match which would have been very well received by her parents!)