Bubbles in oil sight tube

Hi all,
my son took my bike out for a pretty good flog yesterday and when he came back I took a look at the oil level since it was nice and warm. I saw these bubbles in the sight tube (picture 1). After about a minute they pretty much went away (picture 2). Anybody know what caused this, I have never seen it before. It looks like it may have sucked some air or something. I don't see anything obvious or if it is even an issue. Any input is appreciated.

The oil pumps dump the oil at high speed in the tank while you ride the arse of it, hence the bubbles.
It has to do with oil quality too, some oils foam up easier than others.
If it has white foam in it and it stays that's the time to panic as it will have water mixed in it, that point to water pump seal problems.
Just check for that, if no foam after a few minutes it's fine.