Two reasons, I think.
First, Widow's playbook is seduction, martial arts, stealth, and reason. Hulk can't be seduced, punched out, bargained with, reasoned with, or tricked (not with words, anyway; he's easily distracted). No one else on the team, and very few individuals anywhere, are totally immune to her skills. Against Hulk, her intellect, beauty, and years of training are meaningless. She's got nothin', and she knows it. She does not like being helpless.
Second, she's been to the Dark Side. She keeps herself on a tight leash because she knows how bad she can be, uncontrolled. Her actions are only as good as her control. Same with Banner, except he's got it worse. Eventually, inevitably, despite his best efforts, he will Hulk out again. Every time it happens, lives are lost, and there's millions in property damage. I think Widow feels sorry for him, and sees herself in him. If Banner can lose control and become a monster, so can she. The fact that she's already done it only justifies her fear.
First, Widow's playbook is seduction, martial arts, stealth, and reason. Hulk can't be seduced, punched out, bargained with, reasoned with, or tricked (not with words, anyway; he's easily distracted). No one else on the team, and very few individuals anywhere, are totally immune to her skills. Against Hulk, her intellect, beauty, and years of training are meaningless. She's got nothin', and she knows it. She does not like being helpless.
Second, she's been to the Dark Side. She keeps herself on a tight leash because she knows how bad she can be, uncontrolled. Her actions are only as good as her control. Same with Banner, except he's got it worse. Eventually, inevitably, despite his best efforts, he will Hulk out again. Every time it happens, lives are lost, and there's millions in property damage. I think Widow feels sorry for him, and sees herself in him. If Banner can lose control and become a monster, so can she. The fact that she's already done it only justifies her fear.