This phrase describes a situation that sometimes happens. It uses the future tense "will", but it doesn't describe something that happens in the future. Instead, it's something that has happened in the past several times and might happen again. Here are some other examples:
Sometimes I'll be sitting in my office and Nadine will yell out a question to me from the office next door.
You know how sometimes you'll be driving along and another car will pull out in front of you all of a sudden? That makes me so annoyed.