I've never used BURST before but I have some coins on Poloniex and am familiar with the hard drive mining concept.
I believe BURST works in the same way as NXT (which it is based on) in that the passphrase is basically the private key.
A private key is all you need to "own" an account so anyone has access to the Bitcoin wallet with the private key corresponding to the letter "a" for example or for single dictionary words like "cat" or "dog", and putting coins into these publicly known addresses would instantly result in someone detecting it and sweeping them to an address that only they control.
There are people out there who have enormous lists of such Bitcoin addresses with easily guessable private keys. Likewise with NXT, there are people who have enormous lists of NXT addresses with such passphrases. They continuously look for incoming transactions and will try to snatch the coins into their own wallets before anyone else does.
You probably weren't targeted specifically. Even if you have top-notch security, if you have an easy-to-guess passphrase then eventually someone will find your coins and take them.
I'm surprised that someone has gone through the trouble of doing the same for BURST though, since its market cap is so low. I suppose it's possible that there was a manual passphrase collision as well - i.e. someone else somewhere in the world chose the exact same passphrase as yours but given the small size of the BURST community and the fact that the coins were taken immediately afterwards, that's probably quite unlikely.
12 random words from the dictionary is a good passphrase and is what NXT uses. A 35+ character passphrase with letters, numbers, symbols, and upper and lower case characters should also be safe. When in doubt, it's probably best to just use the passphrase that the wallet automatically generates for you.
Yeah, i underestimated it as all over 22 year, i am a very security concern person, i dont simply download harmful links, do malware checks
My passphrase is to easy to get bruteforce
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Yeah, i am making a real big mistakes here
Anyhow, really thanks for the help and explanation here