It's important to remember that peace talks for the Vietnam War had begun as early as mid-1968. However, the North Vietnamese never really wavered from their plan to reunify the country—in the wake of the defeats during the 1968 Tet Offensive, the 1972 Easter Offensive and the resulting ceasefire in place of the 1973 Peace Accords, the North Vietnamese had decided to preserve its forces for a conventional attack, planned for 1976. COSVN, the NLF HQ, issued a directive to halt participation of NLF forces in conventional attacks until a future campaign.

So in short, coming into the accords, they were already thinking about the preservation of forces for a conventional attack. However, the reductions in aid to South Vietnam in 1974 are what truly accelerated Le Duan's plans for a renewed convention offensive.

You could also say that the United States were not necessarily negotiating in good faith, as correspondence from Nixon to Thieu indicates he had promised US intervention in the event of violations by the North Vietnamese in order to get the South Vietnamese to acquiesce to what they thought was a bad idea. Though they'd largely be validated in their concerns, the US were pushing the accords in order to rapidly withdraw their combat ground forces due to the political situation both in Congress and in the public at large.