What we know
- Rocks, Whitebeard, Kaido, Linlin are all heading for the center of GV
- Garp is looking for the big shots(near the center of GV) and especially Roger
- Roger went to fight Whitebeard but after Rayleigh saved Shakky, he aimed at taking the treasures(most likely dfs) and leaving
- Rocks is looking to save his wife and son, but his wife will likely die
So they are all nearby, but the treasures Roger is aiming at taking are guarded by the CDs, who are also attempting to kill Rocks wife and son, which would lead to Rocks and Roger crossing paths with each other with this scene
Most likely when his wife will be killed by the CDs, it'd enrage Rocks who will then start/aim at killing CDs with then Garp forced to have to protect them(Sengoku's words saying Garp was forced to protect the CDs)
And thus leading to the decisive battle between Rocks, Garp and Roger. Although, Roger doesn't really have a strong reason to fight Rocks here, he is aiming at the treasures and leaving, so unless he did it to help Garp(suggesting he was in a difficult spot), his reason is still unclear.
Character-wise, they still look like piece of shit though since CDs are doing a human hunt here, yet Garp will try protecting them despite saying he doesn't want to work for CDs? Truly hypocritical