In Old English, there were three simple pasts of "sink": "sank" or "sonk" (!) if the subject was singular, and "sunk" if the subject was plural. Languages like to simplify things, and since "sunk" had always been the past participle of "sink", it's not surprising that by the 1500s it had beaten out "sank" as the past tense too. That great British authority on the English language, Samuel Johnson, stated in his dictionary in 1755 that the simple past was "sunk" and that "sank" was archaic (in fact the term he used was "anciently"!). I'm not quite sure why "sank" was resurrected, but I suspect it might have been on analogy with the forms of "drink".
Like "sink", "drink" had more than one simple past in Old English: "drank" if the subject was singular and "drunk" if the subject was plural. But the language took the opposite tack from what it had done with "sink": from about the 13th century on, people used only "drank". But then in the 16th century people started to use "drunk" for the simple past instead, just as they were using "sunk" as the simple past of "sink". So, as you can see, both were possible, but finally "drank" won out.
If "sunk" was the standard past tense of "sink" at the time that North America was settled, as Johnson's dictionary suggests, this would explain why it has survived in North America. But the British obviously changed their minds about it and re-adopted "sank". I always like to say it's not the Americans that corrupt English, but the British!
In Canada we tend to be caught between the two standards. In the Canadian Oxford Dictionary we gave both "sank" and "sunk" as the simple past of "sink", with "sank" first, as it is more common. But "sunk" is not wrong. So, one less thing for you to shudder about.
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