I don't know why your avocados were stringy, but if you can figure this out, it might explain why the fresh peaches we buy in season are often stringy, mealy, and tasteless. Even though they have colorful skin, and they look ripe.

Stringiness in avocaos is purely a function of variety. The stringiness is mostly a trait of Mexican type avocados. I've never heard of a "Don Gillogly" avocado variety, it's not listed in the UC Riverside collection either. Chances are this is a trait of this variety. I did do a Google search and found some info on it, it is in fact a Mexican type, and it appears to be a random seedling, so chances are, you get to deal with stringiness in this variety.

See UC Riverside Avocado Collection.