It is said that Senju shinobi received physical strength from the Six Path of Sage. The first Hokage's wife was from Uzumaki clan. Are these the reasons why Tsunade had monstrous strength and was able to use sealing techniques?
I would say partly yes and mostly through training. She developed Chakra Enhanced Strength which allows her to increase the strength of her attack tremendously. It's proven that through training even Sakura is able to show such strength in her attack.
It should be noted that unlike Sakura, Tsunade herself already possesses incredible raw strength, capable of crushing stones in a single hand with nothing but pure muscle power. This strength is later increased to monstrous levels, when Tsunade uses this technique, allowing her to perform feats like lifting and swinging Gamabunta's tantō, creating giant craters and fissures in an area by simply striking it with either her heel or finger, destroying Madara Uchiha Susanoo's ribcage and its sword, and sending opponents flying with a flick of her finger with relative ease.