Main reasons for insane level of power shown
Non-Saiyan mindset
Broly is the only saiyan known to have a peaceful mindset. This could cause a S-Cell count increase of unknown proportions. The saiyans from Universe 6 are actually the only one side of Broly. And their overall power seems to be lower. These saiyans in question have a battle lust, despite being in peaceful times. Broly does not wish to fight, and never had an actual battle until he fought Goku, Vegeta, and their fusion. And the fusion shows his pure mindset. After using a technique that only damages those with evil intentions Broly took no damage. This does not mean Broly is too strong for it at the time. But, as Goku says to Broly afterward, he saw that Broly was good at heart during the fight. Through Gogeta we learn that Broly only fights as a last resort, unlike the rest of his kind.
Zenkai boost
This is what happens when a saiyan gets near death or is revived from death. If you think that a peaceful mindset is the only thing Broly had going for him. Surprisingly, his power level when he was found by the Frieza force was only slightly higher than at birth. Due to only having sparring sessions with his father, he didn't get pushed far enough to rise much higher.
So I'd say, his power level at the time was around 12,000. Which, when compared to Goku and Vegeta separately could have meant certain death. But, as a saiyan, during the fight, his body gained power with each blow dealt to him. After Vegeta went SSG, it would have pushed Broly to equal levels or even beyond. Meaning, Broly's fight with Goku, simply increased the hold he had on it throughout the fight.
Oozaru instinct
Think about the abilities shown by Bubbles (King Kai's monkey). He is oddly communicative, but also has the animal instinct still intact. The Wrath state could, in theory, show what happens when a saiyan embraces the animal side of their nature. As most saiyans have moments like this in their lives, Broly has it built into his DNA since his birth. His first recorded moment of Wrath state was shortly after arriving on Vampa. And, according to Paragus, his tail was removed, despite being a sign of a saiyan's heritage. And Paragus explanation was that Broly was too powerful as an Oozaru to be controlled. I believe Broly's own saiyan biology, despite the loss of Oozaru state, had found another method to release power. The Wrath state is simply put, Oozaru without drawbacks, and the only way to fully utilize the Super Saiyan transformation. It is still confirmed that a child like Broly is only born every thousand years, however the scene that shows the original legendary super saiyan has some odd moments. For one, the form is a mix of Oozaru and Super Saiyan. Secondly, it is the gateway to the true power of Super Saiyan God. Since the original legendary super saiyan was Yamoshi, and his power seemed to grow exponentially as well, Broly will likely be in the same category.
In Conclusion
Thus, due to the constant recurring zenkai boost given to him, and his S-Cell count being massively boosted already by his mindset and lifestyle, could have resulted in a boost to the level of SSGSS in base form during his fight with Goku and Vegeta separately, and his fight with Gogeta pushed him further. Though, without his abilities as the next Legendary Super Saiyan, he currently wouldn't be alive. In the end, Canon Broly simply has become the only being in Dragon Ball to have truly limitless potential because he doesn't want the saiyan lifestyle. Even Gohan will fight, and sometimes is excited at that prospect. But Broly doesn't want to fight, and that's what makes him so strong.