Well it matters where you put the straps of the goggles. if the straps are to high up or low your goggles will fall of. They sould also be separted so one strap is higher than the other.

Goggles for racing have to be chosen/adjusted carefully for the distance between your eyes (nosepiece) and the side of your face.  SOme types are better for some people.  The ones with foam on are not as good as the ones with a sucker like seal.  When racing they are worn extremely tight and you push them in to generate a partial vacuum to hold them in place (after wetting the seal to make it stay).

The other thing is olympic swimmers will be wearing a cap, this helps too.

It is not really a matter of the tightness of your goggles. I am a competitive swimmer and in training I have my goggles on loose with the strap quite low behind my head.

To prevent them falling off when diving, you need to put your hands in a streamlined position in front of you, so your arms are brushing the tops of your ears. You then need to tuck your head under your arms. This may take some practice but it will prevent them from falling off and will make the dive faster due to streamlining.

The answer lies in the head postion of swimmers as they enter the water is the key to this question. As both a competitive swimmer and a coach, I have many a time watched and even myself dived in only to find my goggles around my neck. Which I trust is where the questioners ended up too!

In the case of elite atheletes and swimmers who have mastered the skill of starting their heads are lowered during the streamlined flight and entry. The arms are tight to the head and the head faces down and slightly backwards. This angle of head on entry means that the top of the goggles are essentially shielded from any direct impact. However in less experienced swimmers the head postion in the dive is too high which not only doesnt allow the hips to rise sufficiently but also leads to a direct impact to the top of the goggles on entry and as a result, said goggles round neck effect.

See the attached picture for clarity.

Matt

This swimmer thinks you are either

-over-tightening your goggles and destroying the suction, or

-diving with your face up, allowing the water to get between your face and the goggles, which will lift them off your face.