Scab on mole

Hello, I was in the sun all last week as i was on a trip away and it was very sunny( i had suncream on) . I was sitting out in the sun yesterday and i noticed the mole on my leg looked different. i looked closer. It was a scab. a very tiny one in the middle of the mole. This is not the first time this has happened, although the last time it did was a couple of years ago. so i didnt think they were linked in any way. I have also not fell in any way to cause the scab. Any reason why this is happening or? 

Thank you in advance