As titles says, when spiders spawn fixate a melee and start moving towards him/her, in case the boss is tanked near the side of the 1st “web bridge” then apparently (and i guess depending on the position of the certain melee which i cannot define) the spider loses the “path” to the fixated melee EVADES…and then TELEPORTS under the boss. The boss just kills it enrages and you know the rest.
It is a random and my guess is that it requires a very specific spider “path” as in the most cases there is no problem THOUGH it exists. Investigate it and fix it. Remove any obstacles make the spiders not to evade or at least if they evade to stay on spot and not teleport under boss.