Lelouch's Knightmare is still connected to Britannia's coms. He has the heading and location of all the other Knightmares on the battlefield. It's easy to set traps that way.
They break ranks to fill the gaps and get sucked into an even nastier trap.
Meanwhile, Lelouch's childhood friend Suzaku has miraculously survived the earlier fracas and has been recruited by Lloyd Asplund, head of special weapons engineering for Britannia, to pilot the experimental Knightmare, Lancelot.