AThe stock market fell. So Jeff goes home and shoots his wife, takes his two young daughters, drives away, crashes the car, finds a cabin, tries to shoot himself but realises his daughters will be left to fend for themselves. So he decides to kill them first but Mama lifts him up and breaks his neck before he has the chance. Jeff is also shown to be on some sort of medication, we see him shaking his pill bottle only to realise it's empty. This could possibly suggest bipolar disorder, thus further explaining why he took the actions mentioned above.
AEarlier in the movie, we see Dr. Dreyfuss collecting a box with remains dug up from a defunct cemetery. That box contained the remains of Mama's real baby, the one she lost during her jump off the cliff. The movie shows a piece of cloth hanging from that branch that Mama hit on the way down -- her baby was in there. The archivist who showed Dr. Dreyfuss the box told him (and the viewers) that "a ghost is an emotion bent out of shape, condemned to repeat itself time and time again." That's why at the end of the movie Mama is so intent on jumping off that cliff. Towards the end of the movie, Mama takes over the body of the children's maternal great-aunt Jean who came snooping (she also wanted custody of the children) and uses that body to drive Victoria and Lilly back to the cabin and the cliff at the lake right behind it. Annabel and Lucas make it to the cabin and the clifftop, where Annabel takes out the box, shows Mama the remains of her baby and trades them for Victoria and Lilly. Mama at first seems content with her own baby, but when she hears Lilly resisting Lucas pulling her back to the others, she gets angry again at the people who are apparently taking away the children she's claimed as her own. She beats away Lucas and Annabel and drags the kids towards the edge of the cliff. Annabel fights back tenaciously, until Victoria decides to stay behind. Lilly is happy to go with Mama, and together they jump off the cliff. In a way, everyone got what they wanted: Annabel got Lucas back, as well as Victoria, who she'd grown fond of; Victoria could go back to a normal family with Annabel, who she'd grown fond of; Mama got herself a child, Lilly; Lilly did not have to give up Mama; and Lucas got both Annabel and a foster daughter, Victoria.