One of the interesting aspects of Avengers: Endgame is what Bruce Banner has witnessed in a short time. One aspect that is previously mentioned within the Marvel Cinematic Universe is that while he's Hulk, it's like a lucid dream. He's there, but not really so it feels as though no time has passed when Thor gets him back during Ragnarok. Keep in mind that Hulk vanished at the end of Avengers: Age of Ultron.
This was the starting point as this event terrified him seeing this monster appear that he made, creating a being of immense power (Vision) and then lastly being thrown off a cliff by Black Widow. That was a couple years ago at the time Banner gets back in the driver's seat to then give it up right away to save the people of Asgard. During this time period he's basically awoken to not being in control for a couple years of his life and at the same time being on an alien planet. This can stress someone out and make them angry, which would turn him into the Hulk again.
He then witnesses a planet get destroyed that he's just seen, but heard glorious things about and watches the remaining folks get attacked by some monster with a glowing rock collection. As Hulk, though keep reading. With that being lucid in a sense he might not have a memory, but he certainly is able to account the details to Tony Stark and Doctor Strange. Just as that concludes Earth gets attacked and his green friend won't help so he's stuck in the middle of a battle with nothing. Throwing on the Hulkbuster he's placed in the middle of a war he clearly lacks the skill to be in only to watch many people he knew, as well as new folks that he had just met turn to complete ash.
This leaves him with confusion as to who's new in the Avengers, who he is since the Hulk is fighting him within and of course at the same time dealing with their defeat which cost the galaxy half of its life. When you put this into the context of basically a week, its wild and a rough time for anyone. To clarify any confusion, while time passed to Banner this was essentially a week or so of alert time. You can read our review of Infinity War below and check out the Marvel Cinematic Universe hub for additional coverage of the franchise.