Weather conditions to create significant glacier growth
Hold on, you're talking about growth of the glacier's tongue, not necessarily the whole glacier itself. To my best knowledge, flow dynamics of ice in glaciers still are not wholly understood, meaning we do not really know, why sometimes glaciers seem to grow so rapidly (protruding their tongue).

A prerequisite is the presence of enough ice in the growth-area of the glacier (growth area is the part above the snowline), meaning above average snowfalls in the years or even decades before the easier observable tongue growth. More snow compacts to more ice, generating more pressure on the whole glacier body, finally causing the outflow of the tongue.