In the balance mod the AI keeps 3-5 explorers and surveyors on hand and merely meeting 1 race will not kick if from exploration mode - it's set to > 3. I also think another factor is that survey ships lag behind leaving undiscovered warp points uncovered so the AI thinks it doesn't have anymore systems to explore and the explorer ships just hang out at their target WP. Once the a new warp point is uncovered and it hasn't exited the expand and explore state it will keep going. At least this is my working hypothesis.
