The way people say "byeeeee" in a very squeaky voice is often used in a playful or light-hearted manner. It's a form of exaggeration and can be associated with expressing enthusiasm, excitement, or a sense of humor. The elongated "e" sound and the high-pitched tone are intended to make the farewell more memorable and attention-grabbing.
Language and communication styles often evolve over time, and different trends in speech and expression emerge within various social circles or online communities. This specific way of saying "byeeeee" may have become popular through social media, internet culture, or interactions among friends and acquaintances.
Remember that language is dynamic and influenced by cultural trends, and playful variations like this are just a fun way for people to express themselves and create a sense of connection or amusement in their interactions.