Attacked randomly

[SpiritMuse]

When you're grouped up you're seen as stronger by the game because you are together, and as a result lower level enemies will often be pulled in groups even if some of them are normally non-hostile. If you come in as a high level single player you'll get the same grouping up of enemies as the game determines you to be strong enough to handle that.

[Niwashi]

A mob's icon and level will indicate whether it will attack on sight or not.

If the icon is blue, then it's a passive mob, and won't attack unless it or a fellow mob that it's linked with (indicated by those lines dejiko_san mentioned) is attacked first. Whether you're in a party or not shouldn't affect this.

If the icon is red/orange, then it's an aggressive mob, and will attack any character who's under or up to 10 levels above its own level if they pass too close and within its range of vision. (Apparently mobs are all deaf, though, because so long as you pass behind them and not in front, you can go right next to them and they won't notice. Even mobs that don't appear to have eyes still have forward-facing visual range.)

If you're more than 10 levels higher than an aggressive mob, then it'll act as a passive mob (though still keep that red/orange aggressive icon).