Hamas gutless cowards gone underground, page-75

how did Hamas managed to breach the border fence when its usually so well controlled

They used drones to knock out the power supply to communications.
They used drones to drop explosives on the automated remote control machine guns that Israel uses, causing the associated exposed ammo supply to blow up. Once that protection was gone they opened up the border fence with heavy earthmoving equipment then infiltrated the area with cars and motor bikes. Guerilla warfare.
Presumably they thought that by taking hostages they would have some protection to return to base.

Low tech defeats high tech story at the link.
https://www.engineering.com/story/first-step-in-surprise-hamas-attack-destroy-israeli-communications
The attack started with drones taking out Israel’s communications by dropping improvised explosive devices (IEDs) on electrical generators at the base of a military communication tower near the Be’eri kibbutz.

The resulting loss of a forward command structure combined with the loss of the communication network that made central command screens blank also prevented a concerted counterattack and the ability of soldiers to request that Tel Aviv scramble air support.

Attack planes, based only minutes away, would not arrive for hours, despite social media immediately documenting the attacks.Israeli security, now under the scope for what is being called a massive failure, had warned of heightened communications by Hamas, a precursor to an operation afoot, but the warning was either unnoticed, ignored or delivered to dead soldiers.