How and when does [SWF] happen - xxx bytes after decompression

I have a program that downloads a lot of images, but when there is too much download, it freezes and pushes me away from my program. I tried a lot of tracing from the beginning to the end of the boot process, and everything looks fine and behaves the way I expected. However, every time the program jerks, it displays a couple of hundred of these messages [[SWF] - xxx bytes after decompression ”on my console (almost all at once), and it came out of nowhere when I tried to copy my image upload process with trace instructions, such as starting a trace and ending tracing, these messages do not appear inside, but simply appear randomly.

So my question is: does anyone know how and when this message occurs exactly?

Thanks in advance for your reply.