This explanation is NOT type specific
The rule analogy is probably the best. Hold the ruler near the tips and ask someone to try a defelct the ruler in roll then it's difficult. Hold the ruler in the middle and ask someone to do the same, it is more likely to deflect.
So; As flaps are lowered, the C of P moves inwards (and back). Thus its a alot easier to defelect the aircraft from level flight. For example, the slipstream effect of the propeller hitting the fin and making the aircraft easier to wing drop, generally to the left in a C152
Or on finals with a crosswind where it would be easier for a gust to drop the wing with 2 stages of flap over three.
Generally, flaps reduces lateral stability along the longitudinal plane due to the inward movement of the C of P.