Actually, the real reason/cause:
Teresa LOWERS her yoki back BELOW 10% YR (Yoki Release), instead of raising it above 10% against the now 79-80% YR Priscilla.
Teresa with No YR could just react in time to parry receiving only a cut to her forehead instead of getting killed by the 70% YR Priscilla.
10% YR Teresa was able to pwn 70% Priscilla
10% YR Teresa grimaced/struggled, but did manage to parry ~75-79% Yr Priscilla.
So, with Priscilla now being at 79-80% YR, we can see how Teresa's LOWERING of her YR BELOW 10% YR, was "foolishly-fatal".
BELOW 10% YR (or No YR) Teresa didn't have the Power Level (PL) to be able to react and move in time against 79-80% YR Priscilla, and so she was killed.
0-9% YR = Silver Claymore eyes
10-100% = Golden Yoma eyes
you can see Teresa changing her Golden eyes back into silver eyes in the anime clearly.
but what about the manga?
0-9% YR = Silver Claymore eyes WITH CIRCULAR PUPILS
10-100% = Golden Yoma eyes WITH CAT-SNAKE LIKE VERTICAL PUPILS
this is seen/shown in the manga, if you look closely.
So, Teresa for a fact, LOWERED her YR BELOW 10% in both anime and manga.