There are a couple of things at play, but the main one is a decrease in blood plasma volume. Especially if you completely take time off. You basically diuresed out your fitness.
There are ways to mitigate this during rest weeks like sauna, wearing a lot of warm clothes during exercise, or making sure you have at least one (short) higher intensity day.
How quickly you return to normal training HR values depends on heat, how hard you train, how fresh you are, etc. but should be closer to normal after a week or two with a couple of workouts and long sessions completed. HR values are very individual but 8-9 bpm does not sound excessive, especially if you completely took time off, as noted above.