Trace Inhibition-- Old memories are never lost, they just can't be found.
In the game of pick-up-sticks, if I place a single stick in a cardboard box, I can retrieve it from the box with no difficulty.
If I subsequently place a hundred more in the box on top of the first stick, I may not only find it impossible to retrieve the first, but I may loose all sight of it.
This is analogous to what happens to old memories in Long-term memory. When we have a single experience, it is easy to recall. But once there have been many similar experiences, the first may be out of consciousness.
A child will have learned an entire language system (let's say Russian) by five years of age. But let that five year old come to the United States for the rest of his life and not hear his native language, he will learn the new one totally, and lose any recollection of the first.