I had to look this up as it sounded unusual, but looking at the younh plant it is basically a small trunk covered in long floppy pine leaves/needles that superficially looks like a clump of grass at first. I suspect there's no evolutionary advantage to looking like grass, but storing up energy before growing upwards makes sense if there's a periodic fire risk with each fire risk period being over a couple of years. Also handy for dealing with browsing pressure from particulary hungry critters following a fire.

The long leaf pine has a grass phase where it just looks like grass

Trees are weird, because phylogenetically there is no such thing as trees. As in: there is no single branch of the evolutionary tree where trees split off from other plants and there’s just an entire branch of different trees. Instead, different plants in separate parts of the evolutionary tree evolved into trees, and sometimes back into non-tree plants, and sometime even back into trees again. So a tree that spends part of its lifecycle as grass is par for the course.

Yup. I always wondered what evolutionary advantage resulted in that type of thing happening. Now it feels a bit obvious, although of course nature is crazy complex, so now I wonder what other environmental factors would also lead to that sorta stuff happening. But if there's deadwood piled up to the point it would choke off new growth, obviously don't reproduce until it burns away.