As far as I know, there are four verbs (swear, bear, tear, and wear) whose simple past forms used to be (archaically) sware, bare, tare, and ware; but are now exclusively swore, bore, tore, and wore. There seems to be a pattern here — the simple past of -ear used to be -are, and is now -ore — but I've never heard an explanation of why that change occurred.
I tried graphing sware and swore together (since bare, tare, and ware are all ambiguous) on Ngrams. According to this chart, the two forms were once coexistent, and swore has always been dominant; however, Ngrams has proven to be a less-than-satisfactory authority on issues such as this.
Why did -are switch to -ore?