He knew that Hydra killed Stark’s parents, and he knew that Bucky was Hydra’s top assassin. He couldn’t be sure that it was Bucky himself who did it, but if he’s being honest (and he’s Captain America; he’s being honest) he knows that it probably was.
Don’t a hundred percent know why that made Stark so mad. But I now understand the basic issue at least. (I had to divide my attention during this whole scene so I may have missed some things.)
Also, minor detail missed here, when Cap first says he didn’t know it was Bucky, Stark responds with “Don’t bullshit me.” He’s calling out Cap for lying.
He’s mad because he’s still a spoiled rich kid. Cap didn’t kill his parents, didn’t know anything specific to tell Stark, and there’s probably a lot more things Stark knows that he’s not telling Cap. He’s just being a brat as usual.