This case was also complicated by subsequent non-medically related actions taken by medical personnel which will not be discussed here.
Strictly medically speaking...
Local Anesthetic With Epinephrine
It is unclear exactly when the local anesthetic bupivacaine (Marcaine) with epinephrine was injected into the tonsil region which may help with post-tonsillectomy pain control in the pediatric population. Also unclear is what concentration and how much got injected.
However, it is far from common practice (though not rare) and it is not something I personally inject in the pediatric population for a few reasons in spite of its well-known suspicion to minimize a sore throat after surgery. (There are plenty of papers that report such injection offers NO additional pain control in children. See references below.)
1) The risk of bupivacaine is toxicity to the heart leading to arrhythmias that may ultimately lead to a heart attack.
2) Epinephrine can result in hypertension as well as further exacerbate cardiac problems
Given report of "bloody froth" in the endotracheal tube along with acute onset of tachycardia and hypertension, I suspect the child suffered from pulmonary edema, most likely secondary to accidental intra-vascular injection of the local anesthetic (causing cardiac failure) and epinephrine (causing tachycardia and hypertension).
Given these risks and fact that kids are so much smaller and more susceptible to medication risks, I personally never inject this medication during/after tonsillectomy. Even in adults, I never inject routinely, though I do offer to adults undergoing tonsillectomy (but than I use bupivacaine alone without epinephrine).
The hypertension itself led to problem #2...
Congenital Cerebellar Vascular Anomaly
The child had a congenital cerebellar vascular anomaly that ruptured leading to a life-threatening bleed in the brain.
In and of itself, this anomaly would not have been a problem, but given the reported sudden hypertension, the rupture is akin to inflating a tire with too much air causing it to pop.
Most likely, if the child's cardiovascular compromise never occurred, this bleed never would have happened in the first place.
It's also entirely possible if the local anesthetic was never used, this death would not have happened as well.
Lawsuit claims doctors' mistakes caused Palm Harbor girl to die after tonsillectomy. Tampa Bay Times 6/10/12.
Comparison of clonidine, local anesthetics, and placebo for pain reduction in pediatric tonsillectomy. Arch Otolaryngol Head Neck Surg. 2011 Jun;137(6):591-7. Epub 2011 Mar 21.
Preincisional bupivacaine in posttonsillectomy pain relief: a randomized prospective study. Arch Otolaryngol Head Neck Surg. 2002 Feb;128(2):145-9.
Control of early postoperative pain with bupivacaine in pediatric tonsillectomy. Ear Nose Throat J. 1993 Aug;72(8):560-3.