Something has changed. Their age, their routine in your house, the attention they're getting now versus before. Another animal they're competing with.
Have you been through any major changes in your home that affect the dogs? This includes loss of or addition of a significant other, a new child, a change of food structure.
The most common reason for male dogs changing how they behave toward each other is that one of them has changed ITS role in the pack order. Maybe they've matured physically or sexually.