The growth looks very soft. Have you been fertilising it? I'd just tie it together unobtrusively and clip off any straggly bits. Another thought is whether something jumped on it? And critters climbing over the fence?

i suspect .... this looks like what it looks like when you give a plant a haircut at height.. instead of rejuvenation pruning ...

just below the cuts.. it has sprouted multiple buds.. and growing with such vigor.. what you left behind can not support the new growth ...

i also wonder about too much fert ... i never fert shrubs.. period ... most are aggressive enough without it ...

i also dont understand the 2 pix.. they dont even look like the same plant.. or are they a before and after pix ... this confusion might change my answers above .... what happened to the iron gate in the second pic????

heat has nothing to do with anything ...

ken

Ken, I'm not convinced this shrub has been cut at all. I don't see any evidence for it. If you're growing a fastigiate cultivar why would you cut it off?

Thanks for your advice. Sorry for the confusion. The first photo is now. The second photo is earlier this year.

I installed the trellis recently.

I did fertilize it so maybe all this flopping is just new growth. I will tie it up and hope that it will firm up on its own.

Should I be pruning it if I want it grow tall?

Pruning it won't affect how tall it grows or how quickly. For a fastigiate subject leave the top alone. You can do a bit of trimming on the sides if it straggles. Box is a woody shrub so the fresh young growth should firm up on its own. And it really doesn't need fertilising. All mine are in pots and even they rarely get fed. I wouldn't feed one in the ground at all.