In the meantime, I'll hypothesize
Legal Definition of manifest necessity
: a circumstance (as an incurable pleading defect, the unavailability of an essential witness, juror misconduct, or illness of counsel) which is of such an overwhelming and unforeseeable nature that the conduct of trial or reaching of a fair result is impossible and which necessitates the declaration of a mistrial
This article:
Lead prosecutor delivers brief statement after mistrial declared in Erika Gaytan murder trial - KVIA
Says the aunt was on the stand testifying. "Gaytan's aunt, Maria Elena Ruiz took the stand. Ruiz said Gaytan would never abandon her son. After not hearing from Gaytan, and going to the police, Ruiz was able to speak to Ricardo Marquez by phone. She told Marquez, "Tell me where you left her body.""
The "break" which led to the mistrial happened just after this. Perhaps the parties were expressly told not to allow this testimony? Sometimes testimony is disallowed as prejudicial. Or because it violates hearsay. Or other fun reasons under the law. If the prosecutor or defense was told to not elicit this testimony, and they did anyway, that could be grounds for a mistrial. The judge COULD instruct the jury to disregard, but sometimes that's not enough and appellate courts require a new hearing. So to save everyone time, maybe he declared a mistrial and warned the attorneys to not do it again.
OR.... sometimes during a criminal trial I've heard that law enforcement do background searches on the jurors to ensure they told the truth on their questionnaire and during voir dire. Sometimes things are discovered AFTER the trial starts that would cause a juror to be disqualified. Usually, though, that juror is just dismissed. That usually isn't enough to require a mistrial.
UNLESS.... what if that juror was instead somehow linked to the murder. And was telling other jurors what he/she/they knew or heard second hand. That would taint the jury and would require a mistrial.