With The Last Stand, it’s due to enemy numbers. There is a maximum limit as to how many can spawn at any one time. Also whether they are coded to follow you or not. Some enemies will just stay in a certain area. For example the Drones and Scions at the start of The Surge will never follow you out from the start of the level - only the other end of the act (the ledge they drop down near the saferoom). For The Last Stand, if you run past the enemies, no more will spawn ahead of you as the number of enemies that can spawn is at the maximum limit, plus enemies you encounter are coded to follow you (albeit slowly). That’s why for the solo Gunner speedrun you shouldn’t have Reflect Shredder on as that causes enemies to bleed out and die, which enables some enemies to spawn ahead of you (as the enemy numbers has dropped below the limit).
Thanks for @Bleeding_Pepper & @RumblyMonkey991 telling me some Useful information that usually won’t be asked unless someone are interested in Solo Hive 
Yeah I tried it just now and did it the same way.
I got lucky and there was no Mulcher in act 1 too, but as there’s a shock grenade at the T-junction, you could always use that to stun the Mulcher enabling you to run past or kill it (kill is probably preferable just to get your ultimate recharge up).
To be fair, I didn’t need the Cloak. It only charged up fully as I was running toward the helipad in act 2 and during the running part I only stopped to kill one Grenadier (which I wanted for the stim and it was convenient for me).
I used the Salvo just to kill a couple of Elite Grenadiers and Juvies; the Salvo hits caused another Elite Grenadier to drop their flashbang and flash themselves so I was able to shotgun him super easily for the stim early on. Grabbed an extra Overkill for ammo and when I got to the saferoom I waited. A Juvie and a Grenadier ran after me, so I killed them both, took the Gnasher as well and then continued on. The Shock Grenade in the saferoom was enough for act 2 and I just ran.
Nice! The way you described it sounded like a thriller lol
Oops, I for some reason was thinking of The End when I thought that it had been done. But from reading more posts it looks like Last Stand was done anyway lol.