I voted for Hyp's dad, although there was a none of the above. Everyone listed had plausible reasons for the way they reacted. Mr. Threehorn and Hyp's father both care a lot about their kids.
I think it might have to do with the fact they both lost their original mates before arriving in the Great Valley. My theory on that is based on what happened in the first film and the seventh film.
It probably effected them emotionally too, especially with the way they view others. Mrs. Maia, Ducky's situation, let me put this scenario together.
An animal is guarding her nest and you're all over her nest doing something. How is that animal going to react? Ducky is lucky she got thrown and not crushed like a jellybean.
Plus she would of been better off asking Mrs. Maia if she had seen the twins instead of just snooping around her nest. I mean it's not like Mrs. Maia was asleep for twenty four hours, she could of just fallen asleep before Ducky came into her nest.
While I'm on the subject of Ducky, Ducky's father's situation wasn't anything related to the character himself. Moreover the unfortunate circumstance that befell the late Judith Barsi, who happened to voice Ducky in the original film.
Bron had a valid story, which all goes back to the original film. The land was constantly changing, so he went off to find a new place for him, his mate, and son (who wasn't born yet) to live.
As for Bron not being able to find Littlefoot for years, it goes back to my second sentence, the land was changing. He did run into Rooter, but he didn't know where Littlefoot went afterward.
As for Chomper's parents, sharptooth just being sharptooth is all I can say.

They didn't have that thinking after Littlefoot saved Chomper. The last three is up for debate.