I have seen threads before where when people get their island size to the max... Part of their island gets deleted when other islands spawn near them. Why is that?
Because the skyblock plugin is made as such that anything within the vicinity of another user's island space: (ie. 100 x 100 area around that person's bedrock) will be deleted to make room for the new user.
No. As long as your island is in the boundaries of your limit, there's no chance it'll be deleted. On the other hand, if you DID build last the limit, there's definitely a chance someone could start a new island and cut off your island part that you built into it.