Go Into One's Shells Meaning

Example : 2011 December 7, David Ornstein, “FC Basel 2 - 1 Man Utd”[1], BBC Sport:  Instead of gathering the ball, De Gea poked it away with his foot and Streller was able to place a powerful half-volley into the net.United went into their shells and it was not until the 29th minute that they mustered a response - one from which they should have equalised.