Why does OnTriggerEnter trigger infinitely?

I'm following this tutorial. I have an attacking cube running into a rectangle. I scripted it so that when they collide (OnTriggerEvent), one health point will be removed from the rectangle's health pool. Unlike the tutorial, I do not want the attacking cube to destroy itself.

This works fine, except a few milliseconds/frames later all of the rectangle's health points disappear in an instant as well. There is a slight hiccup between the first point vanishing and the rest of the pool (which happens all at once/super fast).

I feel like something strange is happening with OnTriggerEvent. It's not just triggering on the initial collision, or something else is going on that I can't explain. Do I understand this function correctly or am I misusing it?

Health code:

 using UnityEngine;
 using System.Collections;
 
 public class Healthchange : MonoBehaviour {
 
     TextMesh tm;
 
     // Use this for initialization
     void Start () {
         tm = GetComponent<TextMesh>();
     
     }
     
     // Update is called once per frame
     void Update () {
         transform.forward = Camera.main.transform.forward;
     }
 
 
     public int current()
     {
         return tm.text.Length;
     }
 
     public void decrease()
     {
         if (current() > 1)
             tm.text = tm.text.Remove(tm.text.Length - 1);
         else
             Destroy(transform.parent.gameObject);
     }
 }

Monster AI code:

 using UnityEngine;
 using System.Collections;
 
 public class Monster : MonoBehaviour {
 
     bool triggered = false;
     // Use this for initialization
     void Start () {
 
         GameObject tower = GameObject.Find("Tower");
         if (tower)
             GetComponent<NavMeshAgent>().destination = tower.transform.position;
     }
 
     void OnTriggerEnter(Collider co)
     {
             if (co.name == "Tower")
             {
                 co.GetComponentInChildren<Healthchange>().decrease();
                 triggered = true;
              }
     }
 
 
 }

Now, I already fixed the issue at hand by using a boolean to explicitly state if and when the rectangle can be damaged. So after the first hit, it cannot lose health (for now):

 void OnTriggerEnter(Collider co)
 {
     if (triggered == false)
     {
         if (co.name == "Tower")
         {
             co.GetComponentInChildren<Healthchange>().decrease();
             triggered = true;

But I don't want to use a band-aid if I don't have to. I thought Unity was smart enough to do this logic for me. Am I wrong? Why is the collision event triggering more than once when the objects are just standing next to each other?