The aircraft does not pitch up as you loose airspeed. If in straight and level flight you reduce power, the nose will pitch down. If the pilot wants to maintain level flight at a lower speed he must increase the angle of attack of the wings as the amount of lift is proportional to speed (V squared).
As has been correctly pointed out already - as the angle of attack is increased to maintain the lift the centre of pressure or lift moves closer to the leading edge of the wing. Eventually as speed is reduced further and angle of attack increased to compensate the moving air will no longer be able to flow over the wing surface due to low speed/energy and surface/skin drag and so separate causing loss of lift and eventually stall. In straight wing aircraft the nose tends to pitch down during the stall. In swept wing aircraft the wing tip may stall before the wing roots causing an undesirable nose up pitch (unless design features have been incorporated into the wing to ensure the root stalls first - such as a twist in the wing that means the tips have a lower angle of attack than the roots, wing fences to prevent spanwise flow, vortex generators on the tip surfaces to re-energise the boundry layer of air flow and Krueger Flaps on the roots to encourage root stall first).