1st Theory:
In Halo 5 Guardians, there will be two ending, before the start of the last mission u choose to be either Chief or Locke and that trailer is the final cutscene depending on which character u chose. While this one is more unlikely because how will that carry over to halo 6. It was just my first thoughts.
2nd Theory:
Locke’s one is one that ONI made too try and tell the public that Master Chief is dead and he turned into something he shouldn’t have, While Master Chiefs one is actually what happened.
These are just 2 theory’s i thought of when i watched the trailers, Let me know which one u agree with the most, or state ur own theory’s for these trailers.