FATHER is continually monitoring the structural integrity of the holding cage. Spitting acid would have weakened the bars, but not before the handlers were able to use the nitrogen gas button to incapacitate the xenomorphs. By ripping one of their number apart, they were able to spread acid all around them (and cover themselves in it), hastening their escape.
“There is serious structural damage to animal holding facility number
zero, zero, one. There is serious structural damage to animal holding
facility number zero, zero, one. The damage is sufficient to breach
the security of animal holding facility number zero, zero, one. There
is serious structural damage to animal holding facility number—”
Note that the official novelisation mentions that the Alien that was killed was suffering from a bout of individualism. Despite being able to see the sense of killing itself for the greater good, it was also reluctant to allow this to happen.
The first warrior turned to his closer brother, transmitted
information to him. The second warrior absorbed the information. He
stopped pacing. Together, the two looked back at the third. The
smaller one understood their objective, their reasons, the whole new
concept. He even agreed with it. However, he was also burdened with
his own individuality, and edged back against the wall of the cave