Perhaps some cybercriminals were able to extract your Bitcointalk session cookies and sell them on a darknet marketplace while you were away (this can be mitigated by logging out and back in again).
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Cookie stealers mostly concentrate on Google accounts, especially YouTube but this may be the first time in my knowledge if that is the actual reason for this compromise.
In the recent past, a few accounts were compromised in the same way but they just used to take loans via bitcointalk and let the owners pay it back so possibly the same group targeted the bitcointalk accounts again with much more severe like spreading malware and possibly to attack more users whoever by clicking the link.
So I wonder if this is going to be a big issue if this happens to other accounts too.
Apart from using strong passwords, we don't really have any mechanism to fight against this so probably this can be the reason to implement 2FA on bitcointalk.