You're creating a connection at the class level, but never disposing it, so it's hanging open every time you open and close your form.
The same is true for the SqlCommand object you're creating up there too.
AWAYS create the connection object when you need it, as late as possible, and create your command object, set it up and execute it, then you MUST call Dispose on the command and connection objects when you're done with the command, disposing them as early as possible. This is what the "using" statement is for:
using (SqlConnection conn = new SqlConnection(connection string)
using (SqlCommand comm = new SqlCommand(statement)