Why is the trailing edge sweep angle smaller at the wing root?
There are two main reasons:

Swept wings suffer from interference at the center of the wing. If wing taper and sweep remain constant over the full span, the lift distribution will show a distinct drop at the center. This is called the &quot;Mitteneffekt&quot;. The remedy is to locally increase the wing chord near the root and to decrease the wing's sweep angle. This, together with a change in airfoils and local incidence, &quot;fills up&quot; the dimple in the lift distribution and reduces induced drag.
Large aircraft need heavy wing spars, and increasing chord near the root gives the spar more height while the relative thickness of the airfoil can remain unchanged. A higher spar is structurally more efficient, so increasing its height at the root saves weight. Since the relative thickness determines the maximum operating Mach number, it cannot be increased locally without hurting the whole wing. The better alternative is to disproportionally increase wing chord at the root.

The unswept trailing edge near the root is a consequence of Boeing's flap track mechanism. Boeing Fowler flaps move perpendicular to the hinge line, and an unswept hinge line avoids lateral movement of the flap. Another consequence is a gap between the inner and outer flaps in order to allow both to move back without colliding. Boeing fills the gap with a high-speed aileron. Also, this gap avoids that the flaps protrude into the hot, turbulent exhaust stream of the jet engine.
Airbus flaps move in flight direction and leave the sweep angle of the trailing edge free. Also, they don't need a gap between the flap segments left and right of a sweep change of the hinge line.