In Thor Ragnarok we find out Hulk has been in control of the body for a long time. Does anyone remember how long??
When Banner finally emerges he talks about how scary it was to be suppressed and gone that long and he feared he would never come back and he fears the next time Hulk emerges he might not come back.
Banner has, at many times, suppressed the Hulk for years. I am pretty sure in the first Avengers movie when Natasha found him they said they had not seen the Hulk for two years.
So I think they are going with the angle that Hulk is pissed about being suppressed and only being summoned to save Banner's ass and therefore he is throwing a tantrum of sorts.