Sent by KlaudiaEditor: My guess is that you're getting either wild yeast or bacteria in your yogurt as it cultures. To prevent this, make sure to heat your yogurt to around 200°F, then cool it to around 115°F before adding the culture. It also helps if you can keep the yogurt above 100°F as it cultures.
Readers, what other theories do you have for this stringy yogurt?
Related: How to Make Homemade Yogurt
(Image: Emma Christensen)