Drop down options appear on web

I have published a course that users do online. There are a few numeric entry boxes and drop down boxes have started popping up offering the user options that they have previously entered - like sometimes happens if a website asks for your name etc. I have tried to attach a picture of an example. This wouldn't be a major problem but if they click on the box, it stays on the screen and doesn't go away until they move onto another slide - something which they don't really do to often as I use a lot of layers in the course. 

It doesn't happen for everyone so I assume that it's related to a setting on people's internet browser but I have no idea what setting it would be and so cannot tell people anything about it - other than avoiding clicking on it - which isn't that useful.

Does anyone have any idea why it happens and what I can do?

Thanks!