To prevent dust accounts from bloating the chain state, Polkadot stipulates that if the balance of an account is less than 0.01 DOT, the remaining DOT will be burned. To avoid this, please make sure that the remaining DOT in your account is always greater than 0.01.

Further reading: Polkadot Protocol Information

Note: If your address is using DOT network's staking service, you don't need to worry about this issue.