By leaving it in too long. Bacteria then have a chance to grow and it's the bacteria that cause TSS. If you change tampons at least two or three times a day you should be OK. The problem first occurred with super-absorbent tampons that some women left in for days at a time. Those tampons are no longer made. It seemed like a good idea at the time but had unintended consequences.
You can get Toxic shock syndrome if you leave a tampon in for too long , it is caused by bacteria that gets into the blood stream and can be very dangerous.
Best wishes
Angie
You can also get it from using too high an absorbancy as well. That's what happened to me. (super absorbancy is still made though. In fact I'm looking at a box of em) just change often and don't use anything higher than what you need to ad you should be fine.