All the source says is:
c 8 William, 1689, d 1712. This was a "Bonny Brier-Bush" case. The young man was greatly beloved and sorely lamented. Jonathan, above, notes that he named his first son, born later in 1712, for this young cousin.
I did a Yahoo! search and found a poem that seemed to be about some young persons fooling around under the bush, then worrying who was going to love them in the future. I've never been good at translating poetry and stuff, but the poem left a lot unsaid. Did someone kill William for fooling around with their daughter?
There was also reference to an old Scottish story and another poem by Burns. This one was more about a young couple from different classes.