I graduated with a computer science degree a few years ago, and yes, at least a third of the TAs were international students (mostly Chinese). However less than half the graduate program were American, so they didn't have nearly as many students to choose from. In the freshman and sophomore classes, when the vast majority of students dropped engineering, all the professors and TAs were American or spoke excellent English, so this wasn't an excuse. Engineering is hard, very hard for some, and isn't culturally valued in our society. We lost nearly 25% of the incoming freshman engineering class to business or arts and sciences. Some weren't adequately prepared in high school and others just wanted an easier degree. Not the TAs fault.