In some fanfictions, Draco might save Hermione from Bellatrix out of a new - found sense of morality. Maybe he has grown tired of the Dark side's cruelty. He could see Bellatrix about to harm Hermione and use his knowledge of the Death Eaters' ways to create a distraction. For example, he might pretend that there is an intruder in another part of the manor, causing Bellatrix to leave Hermione momentarily, giving Hermione a chance to escape or for Draco to help her further.
Draco might be motivated by jealousy. He could be jealous of Ron and Hermione's friendship or a possible romantic relationship between them. So he tries to harm Hermione out of spite.
Well, perhaps Snape uses his position as a teacher to step in. He could have been observing the situation secretly and when Draco was about to do something harmful to Hermione, Snape intervened with a firm command or some magic spell to stop Draco in his tracks.
Draco could be seeking redemption. He may be tired of being on the wrong side of the war. Saving Hermione, who is a symbol of the good side, would be a significant step in his journey towards redemption. Additionally, he might be influenced by his friendship or alliance with some of the other characters who are on the side of the Order of the Phoenix. They could have convinced him to do the right thing and save Hermione from Bellatrix.
Well, in some versions, Fred might use his quick-witted magic skills. He could cast a distraction spell to draw Bellatrix's attention away from Hermione. Maybe a spell that creates multiple illusions of himself, confusing Bellatrix long enough for him to grab Hermione and disapparate them to safety.
One possible plotline could be that Harry uses a powerful spell he's been secretly practicing to stun Bellatrix just in time as she's about to harm Hermione. For example, he might use a modified version of the Stupefy spell that's more powerful. Another could be that Harry manages to create a diversion, like summoning a horde of magical creatures, which distracts Bellatrix long enough for him to rescue Hermione.
In some fanfictions, Ron saves Hermione from Bellatrix in a more psychological way. He manages to get into Bellatrix's head, using a rare mind - influencing spell. He makes Bellatrix see her worst fears, which causes her to drop her wand and flee, leaving Hermione unharmed. This shows that Ron doesn't always have to rely on brute force to save Hermione. It also adds an interesting layer to his character as someone who can be clever and resourceful in different ways.
There are fanfictions where Ron teams up with other characters to save Hermione from Bellatrix. Maybe he gets help from Neville, who has also grown to be quite powerful. Together, they manage to outwit Bellatrix. Ron could use his strategic mind to create a diversion while Neville sneaks up on Bellatrix and disarms her, allowing Hermione to be freed from whatever spell or trap Bellatrix had her in.
One possible scenario is that Snape steps in during a confrontation. Bellatrix is about to harm Hermione, and Snape, using his quick reflexes and advanced magic, stops her. He could be acting out of his own sense of right and wrong, or perhaps because he has a hidden agenda that involves keeping Hermione safe.