It's not a common mistake but OP was just too naive that time or perhaps he didn't know what was the purpose of having a seed in our wallet.
If I want to answer that question of OP, maybe leaving your crypto in exchanges will be counted as a lost Bitcoin due to hacking incident. But I can name most common problem of losing Bitcoin and those are:
–leaving your coins in the exchange
–giving/showing your own private key/seed to others(just like what happened to OP)
–investing in a HYIP/pyramid scheme/doubler etc
–sending your coins to the wrong address
–forgetting the seed key to your wallet or you misplaced those keys or possibly gone etc
Expensive way to learn from things but a simple reminder that we should all invest tume to know things where we would be engaging ourselves with. Just think of cautious investors knowing the risk but have still suffered from concerns of the like. We are using online technology meaning, this technology is also prone to negative tendencies of keeping an asset. So better prepare onself before starting; dedication, orderliness, and other positive characterists should be met before investing. Having a strong foundation won't put you in trouble.
And with regards to these listed possibilities or causes, there are more for sure. And to add; malwares on devices such as phising sites could also put your assets at risk. Then, if there are many risk involved why are people still continuing their investment, is probably and more likely because of profit potential; high risk and high reward. Also, these possible causes could be aided by being fully aware of what you are actually doing.