Is this a common play say in bridge or some other euchre where players just randomly throw their last card in. I notice its commonly done during a lone hand but also on regular play and I have to always defend that it was my lead.
It's common in all trick-based card games; since your 'move' is forced (you have to play the only card you're still holding), you're not giving away information to your opponents (or your teammate). By just playing your card the very moment the second-to-last trick is taken, you speed up the game.
Players who do this know who is in the lead; the player who took the second-to-last trick, not the one who throws his/her card first. They assume the other players know it, too. Even more: most of the times they probably already know who is taking the last trick, which can often be deduced by remembering cards played during previous tricks. E.g. it might be clear that there is one trump card left and only one person can have it.
If it's confusing you or one of the other players, e.g. because you/they are relatively new to the card game, just ask the out-of-order-throwing players politely not to do so. Card games are a social activity and if you ask nicely, they're likely to comply.
It is not that everyone throws the last card it is usually one person doing this and with people trying to learn and older people who can't see or hear that good as well as other issues like concentration etc.
We all can't be experts or play super fast. Older folks don't like to play like that and people who are just learning don't get that way of playing until they understand better. It's ok to play like that at home but at a tournament or even regular play there are rules to follow. It may sound odd to an experienced player, but you have to remember there are new players or older players that are not on the fast track.
Most people don't play like that so when one person throws it in. I can see if some throws in the right bower or gets last trick but this is not a euchre where once you are euchred, they throw the hand in.