It's not about actual fire, but about fast transmission of information or trends. When he announced his retirement, it spread like wildfire across the internet.
Pro Tip 2/3
Negative News
Often used for controversial or shocking updates that grab people's attention. The scandal spread like wildfire through the high school.
Pro Tip 3/3
Emotionally Charged
Can carry a sense of urgency or excitement related to the shared information. The rumor of the bonus spread like wildfire among employees.