To let a DEX swap your tokens, you first approve its contract to move them. Many interfaces request an unlimited allowance to save you gas on future trades, and that permission persists indefinitely.
The risk is not the swap you are making, it is the future. If that contract is upgradeable, compromised, or was malicious to begin with, the standing approval lets it drain the whole balance without further action from you.
Practical routine: review approvals periodically on a block-explorer or revocation tool, revoke anything you no longer use, prefer exact-amount approvals for large balances, and use a burner-wallet for unfamiliar apps. Revoking costs gas but is far cheaper than the alternative.
Related: burner-wallet, wallet-drainer, erc-20, admin-key-risk