It looks very healthy Julia. Pinus Mugo does need free drainage. None of my pinus are dropping leaves right now, so maybe its just a bit waterlogged. This winter...well...enough said!

As it appears to be all round I suspect this is down to poor drainage. try and improve the soil if you can add extra grit. these brown needles will drop. Hopefully as we move through spring into summer it will stay healthy for you.

Pines normally shed their inner needles where the sun can't reach. Your bush looks very healthy. Now if the outer needles turn brown or the upper new needles turn brown, that's cause for concern. Otherwise you are fine.