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I can't tell you exactly what happened to let this particular breach happen, but from what I've seen myself and what I've heard from family:

1. There are many security checkpoints like the one where my cousin got stabbed (in his vest, he's ok!!!) Palestinians are guided through essentially storage containers and checked by members of the IAF on their way to work or whatever reason they have to visit what we'll agree to call Israel merely for these explanations. While heavily guarded, these points would be incredibly easy to breach if you're willing to sacrifice enough lives through drastic enough of measures.
2. While many are heartless regarding the situation the Palestinians have been put in, the Israelis aren't totally emotionless. Some parts of Palestine are quite cultural and beautiful, so the walls are more like fences so that the Israelis and tourists can appreciate the aesthetics created by the oppressed population.
3. Remember the rule of 80/20: the top 20% of the areas get 80% of the resources due to wealth inequality. While many areas have perfectly adequate infrastructure, many places have over-the-top security and infrastructure, leaving some places to be underfunded and dilapidated. It's not totally unbelievable that people could get over, under, or through the walls.
4. People get into Israel "illegally" all the time. These holes in the security are quickly found and patched up because the person who crossed the border is quickly collected and questioned. I'd reckon that if more patient members of the population took the time to construct more effective modes of crossing the border, they wouldn't be discovered due to ignorance, allowing a large crossing briefly. When you're relying on opposition telling on themselves, you don't know what they don't tell you.
5. Sudden strength in numbers with little broadcasting would be enough to overwhelm some crossing areas.

Ultimately, something like this was bound to happen eventually, primarily due to the leadership of ~~king~~ Netanyahu and the poisoned minds of the Israeli population. I'm not saying that all people in Israel are inhuman, but the constant barrage of belittling talking points about the Palestinians have caused even some of my more sympathetic family members to begin believing some of the propaganda that allows atrocities against other humans to go relatively unopposed. The way that they've learned to justify these things to themselves is shocking to me, especially when they follow up toxic sentiments with, "most Palestinians are incredible people and all of Palestine deserve peace."

Also, if you'll allow me to put on my tinfoil hat for a moment: with all of the egg that's been all over Netanyahu's face recently, a war is a really great way to garner support and deflect attention from his scandals. I wouldn't be surprised to eventually learn that there was plenty of intelligence about this attack and they were dismissed as lacking credibility and whatnot. I'm not saying that I 100% believe that Netanyahu possibly had a hand in these attacks happening, but I do know that he's been talking about, hoping for, and probably needing a war for quite some time to empower his corruption and further genocide a population for which he harbors incredibly disgusting feelings.