The 't' quantity you specify in Vector3.Lerp is likely larger than one, causing your first problem. Secondly even if you make it smaller, your cards will never reach their destination because the 't' value will never equal 1. They'll approach it quickly at first and then slow down, but never reach the end point.
You'll need to change your functions to store the end positions you want, and also how far into the current transition we are. To do this I'll create two new variables
float cardTransition = 0.0f;
Vector3 cardEndPositions = new Vector3[cards.Length];
Then you can change your mover function then to look like this
public void mover(Vector3 endPositions, float transition)
for (int x = 0; x < cards.Lengh; x++)
cards[x].transform.position = Vector3.Lerp(cards[x].transform.position, endPositions[x], transition);
When you want to start a transition now, you can intialize the cardEndPositions like this
for (int x = 0; x < cardEndPositions.Length; x++)
cardEndPositions[x] = new Vector3(Random.Range(2.5f, 15.5f), 1.5f, Random.Range(-5.2f, -13.5f));
Also make sure to update the cardTransition variable in your Update loop while a transition is occuring, add a small value to it each time step similar to what you had in the original call to Vector3.Lerp()
if(cardTransition < 1)
cardTransition += CardMoveSpeed * Time.deltaTime;
CardMoveSpeed should be much smaller than what you have it, since currently this product is greater than 1. Tune it a bit, or use the debugger to see what values would be reasonable in here.