I surmise attempting to assign blame is a way for Mr. Threehorn to assert some sort of control, since he otherwise feels helpless. Cera could be anywhere and anything could be happening to her, and there’s little if nothing he can do to keep his daughter safe.
I believe Grandpa Longneck reached this breaking point because unlike the last time Littlefoot’s gang went gallivanting off, there is now no consistent location where the kids could reunite with there families.
Everyone is always on the move, hungry and tired from long walks and inner herd spats. Those spats that threaten to split the herd up caused the gang to leave in the first place. The herd recently lost track of the gang’s whereabouts. No one knows about their condition or how they could track down the herd.
It’s everyone’s low point and combine that with some reminders of his own daughter’s passing, it’s no wonder Grandpa Longneck’s starting to feel the strain.