I hate saying this but I am going to guess that he pleaded guilty because the government has overwhelming evidence which backs up its case, and he is looking for a lenient punishment.
No. He is pleading guilty because there is no expectation of justice from the United States military. The best he can hope for is to just get this over with as quickly as possible so he can appeal in the civilian courts the fact that his due process and 6th Amendment rights were grossly violated.
He pleaded not guilty to a second charge of missing a flight he was required to be on. His court-martial at Fort Meade is continuing on that count.