Probably the problem is that you trimmed it in March, and a bit of frost got in to the cut bits and caused the tips to die back. But it takes a very long time for dead conifer to turn brown so you only just noticed. Basically the brown bits will probably stay brown, because new growth does not occur when there is no living fronds on a branch. Basically what you do in such a case is take bits of the adjacent living bits and twine them into the dead areas, a few inches back from the surface, so that you can trim them in future without cutting them off. Over time they will extend can become the green for those dead areas, and might eventually totally cover it.

In future confine your conifer trimming to May to September. It is normal to do it once a year about now.