Warning! Spoilers ahead for Dragon Ball Super chapter 77!
The latest chapter of Dragon Ball Super almost broke the internet when fans learned how Goku's father Bardock saved his son's current nemesis, Granolah the Cerealian. And the reason why Bardock did can be traced back to what prompted his actions in the film Dragon Ball Super: Broly. It's all because of his love for his son and wife — that, and a bit of guilt.
Granolah is the sole Cerealian survivor of a Saiyan invasion that was orchestrated by none other than Frieza. For decades, Granolah never knew how both he and his Namekian caretaker Monaito survived, until Monaito finally tells him the truth at the 11th hour. In chapter 77, Granolah is in the process of killing Goku and Vegeta to avenge the genocide of his people. And Monaito, who sees the resemblance between Goku and Bardock, wants to make sure that Granolah doesn't accidentally kill the son of the man who saved him and his mother.
In the fan-celebrated flashback, the Saiyan Bardock stumbles upon a young Granolah being shielded by his mother at the tail end of the Cerealian genocide. But instead of seeing two enemies he must kill, he sees his own wife Gine and son Kakarot (later known as Goku). Bardock then recalls the first time he laid eye on his son. He'd just returned from a months-long mission and missed Kakarot's birth much to Gine's annoyance. And as Bardock tries to defend himself from his angry wife, he spots his son and is so startled that Gine can't help but comment. Regardless, the association that Bardock makes between his own family and his Cerealian victims is so strong that when a Saiyan comrade approaches him, Bardock hides them from him.
Although Bardock initially saves the young Granolah and his mother, he is obviously not pleased with his actions. He isn't able to stop himself from expressing his frustration out loud and later asks himself if there's something wrong with him. But he still protects them regardless. It's obvious by how Bardock reacts that this isn't normal behavior and that he would have most likely killed Granolah and his mother without hesitation if he didn't have a family of his own. But based on how much of an impact seeing Kakarot had on him, it's not surprising that Bardock would feel compelled to protect another child, even the child of an enemy. It doesn't help that Granolah's father isn't there to protect his own son, forcing his mother to act alone, which most likely reminds Bardock of his own absence. It's not far of a stretch to assume that, while he might still be riddled with guilt for missing Kakarot's birth, Bardock might want to make up not being there for his family to protect this one.
Regardless, it's obvious that, even before his retcon, Bardock loved his son enough to go out of his way to ensure his survival right before his own death in Dragon Ball Super: Broly. Since he cares deeply for Goku, it's not out of the realm of possibility to assume that his feelings would affect and cause him to react in other ways beyond just attempting to save him. And that's why Bardock protects Granolah and his mother now.