Obsessions or trends among teenagers can emerge for various reasons, often influenced by pop culture, social media, or peer influence. It's possible that there was a specific online challenge, meme, or popular culture reference related to pineapples that gained traction among teenagers for a time. However, such trends can be fleeting and constantly changing.
Keep in mind that teenage interests and obsessions can vary widely across different regions and time periods. If you're curious about a current trend or specific event related to pineapples, it would be best to look up recent information from reliable sources or observe discussions on social media platforms to understand the context better.