Why melting temperature of ice drops under increasing pressure?

Answer 1

My recollection is that it has to do with ice being less dense than liquid water. As pressure increases, liquid water becomes a marginally lower energy state.

To freeze, water will have to do work of expanding against the increased pressure. This means that the melting temperature of ice lowers slightly as pressure increases.

Water is unusual in expanding as it changes phase from liquid to solid. Most other materials are the opposite and their melting temperatures increase with increasing pressure.

Answer 2

http://www.lsbu.ac.uk/water/phase.html

Here's the graph for the person with a good technical explanation to use.