Race relations and general distrust were core themes across all three previous games, with the ultimate goal in ME3 being to unite every major race in the galaxy under one cause. This particular theory goes that the Andromeda galaxy have been almost completely unaware of the goings-on within the Milky Way, and as such have only heard bits and pieces about what a 'lowly human' (remember, humans are the last species to join the approved Citadel Races) has been doing to the galaxy or universe itself. A rumoured villainous force we're supposedly going up against are the Khet, a powerful sect of enemies who are seeking to establish dominance by opposing all others. There's also the mysterious Remnant (essentially the new Protheans) whose influence hangs large over the new galaxy, with many stories motivating both the Khet, remaining Remnant and other races to establish what a threat humankind poses to their wellbeing. Also, as mentioned earlier, humans were accepted as a 'trusted' race to the Systems Alliance until they became a Citadel Race in 2165, before discovering Reaper tech and triggering the mass genocide of all species within their own galaxy not 18 years later. It's not a stretch to assume that we humans are actually to be feared, judged and analysed, should any of us then arrive in another galaxy.
