It's the lag issue, if you don't get near him, you may be too late to get beneath him when he use that smash.
I'm not talking about lag. Lag is random.
Let's say a Sniper is shooting at the ape. Shooting at his face if you want, which means the Sniper is in front of him but safely at like 20m. Ape starts pounding his chest. Sniper runs and keeps being in front of him. Smash. Everybody on falls on the ground. Where is the Sniper ? In front of the ape and not safely at 20m. Sometimes the boss will jump to the pipe immediately after the smash, sometimes he wont (dont know that works tbh). If he doesn't, the Sniper will most likely get cleaved.
When you run to the boss if you're heals or dps, stay looking at his butt, after the smash when you fall, you'll be at his back, so no risk of getting cleaved. Not everybody does this.