Despite my best efforts, I can not find the answer specifically for "a" online. For the rest of the letters of the Latin alphabet, I can see the connection between the different forms (except for the "long s"), for example it is clear how the middle line is connected to the top line in "e", but "a" seems to make no sense. Even weirder considering how it looks very similar in Cyrillic and Greek.
My best guess is that the "Ʌ" shape was written from left to right, then up and left again to make the middle stroke, which extended into a full circle and the left side of the "Ʌ" retracted a bit to make the hook found in Cyrillic and sometimes Latin, which was lost in Greek.