What caused Opal to forget about Winn-Dixie

Opal forgets about Winn-Dixie because she's so preoccupied with saving her outdoor party from a sudden thunderstorm. She and Gloria Dump threw the party together, and the impromptu downpour threatens to destroy everything. When the rain hits, Opal snatches the egg salad sandwiches and runs them into Gloria's kitchen. Then she tries to take Gloria by the arm and lead her inside. She spots Otis by the garden, gazing at his feet, and beckons him to come inside, too.
It's not until Gertrude squawks "Dog" that Opal realizes Winn-Dixie is missing. In all the commotion, Opal simply forgot about Winn-Dixie who, like most dogs, is terrified by thunder and ran away. Though she's happy that she helped ferry the party-goers (and the food) to safety, Opal feels guilt over not protecting Winn-Dixie and holding tight to him when the storm began.