Clark hears a sniper from afar and steps in front of Bruce to protect him, clearly expecting the bullet to just ping off him. But it doesn’t, and Bruce sees the confused look on Clark’s face before he collapses. The bullet is kryptonite, of course. Bruce has to carry Clark to safety, then has to get that bullet taken out of him. Later Bruce realises he has Clark’s blood on his hands/shirt, and he can’t stop looking at it.

The bullet has leached some trace elements of kryptonite into Clark’s bloodstream, which slows his recovery. Bruce is extremely worried about Clark (trying and failing to conceal it, much to Alfred’s total lack of surprise), and seeing Superman being wounded brings back memories of Doomsday killing him. I would really love to see Bruce sitting by Clark’s bedside a lot (and possibly holding his hand when he thinks Clark is asleep), which then leads to feelings being revealed.