The item was nearly empty and so a fair amount of space inside it with air pressure at sea level. When I unscrewed the top - the roller ball shot out of the base like a ping pong ball
Ahh haa, it's not just me then..fortunately mine was in hand baggage in it's mandatory plastic bag but even so
For the OP
a .5 litre tupperware container 75% full .......
Just like the deodorant container the problem is the air (which is compressible) , not the fluid (which isn't). You see the opposite effect if you seal a water bottle at cruise altitude and then look at it after landiing - a full'ish bottle will retain it's shape, a nearly empty bottle will be crushed and may quite probably have self destructed ( and ruined the contents of your briefcase
). Always minimise the amount of air you "pack" an airtight container....