By Fan theories come in all sorts of forms, but the best ones - well, the second-best ones, after the ones which invent enormous tracts of mythos as scaffolding to explain something which was very obviously a continuity error - are the ones which don't really have much bearing on the plot but just round out a character arc a little more.
Reddit user u/Quilled has one such, taking a tip from the fact that director James Gunn said that Guardians of the Galaxy Vol 2 was specifically about fatherhood ("At its heart, Guardians is a story about families, and if the first film was about him and his mother, this is a story about fathers") and runs with it.
The theory goes that Rocket's decision to run off to apparent safety with Thor during the first act of Avengers: Infinity War - rather than hang about with Star Lord and try to take the granite-headed gauntlet fan on head-to-head - wasn't down to Rocket's apparent cowardice.
The whole Guardians gang helps to raise baby Groot at the beginning of Guardians 2, and in the intervening four years or so between the end of that and the start of Infinity War, they've presumably only got tighter. Knocking about round the galaxy in a fairly poky ship, righting wrongs and covering each others backs and that - it's a good way to bond.
So, when Thor pitches up and tells everyone that Thanos is after the gauntlet and intends to wang it about with merry abandon, what does Rocket do? He makes the best decision for young Groot, making up some specious nonsense so he can ghost the young sapling away from the danger. Obviously it doesn't quite work out as young Groot ends up dusted anyway, but it stands up as a theory - especially after James Gunn tweeted that young Groot's last words to Rocket before evaporating into nothingness translated as 'dad'.
