so the DDer think to himself: "But if i and some other DDs got together and told the boss we aint diggin' no ditches before he give us 10% more,problem solved!"
Not necessarily. There are other workers (illegal immigrants for example) who would be willing to do the job for less. Additionally, the dd would not just be able to get together with some other ditch diggers, they must all unite. In which case the employer cannot afford to hire as many. This in turn displaces the unemployed workers to the non union ditch digging workforce. This creates a greater supply of non union dd's which drives the wages down for the non union dd's.