Spoiler Thread: Avengers Infinity War

I think y'all are to focused on what you know about comic book Hulk and not focused enough on realities of MCU Hulk.

MCU Hulk just got the *** whipping of his lifetime from Thanos. Something he never experienced before. Now he can likely grow into a bigger rage monster and fight Thanos (who would still win due to the stones), but for the first time in his existance, he feels fear, he feels doubt, and he's not sure if he can beat him. We know from Ragnorck that Hulk in the MCU isn't just a rage monster, but a complex being with many emotions.

In that context, it makes perfect sense that he would do what he did (basically hide). He'll no doubt overcome that, but it will take something happening that propels him to overcome his fear. Black Widow's life being in danger would make perfect sense.