mo·men·tum: 1. Symbol p Physics A measure of the motion of a body equal to the product of its mass and velocity. Also called linear momentum.
2.
a. Impetus of a physical object in motion.
1. The fact that the Hulks foot was moving backwards slightly during his battle with Cain in WW Hulk, shows us that the Juggernaut had momentum. Even though it was far less than the momentum that he would have if he was running towards the Hulk. This is still supposed to be within his power set to overcome, and continue onward. As we all saw, he could not, at least not until the Hulk decided to move aside, and let Cain run into the lake bed.
2. The Juggernaut was simply unable to plow through the Hulk at that level because the Hulk was able to overpower or match the enchantment. War was unable to stop Cain's advance in as timely a manner as the Hulk did during the WW Hulk saga. Which should leave most people thinking, that WW Hulk was on a higher level than he was when he was briefly known as War.
3. Now take into account that the Hulk wasn't even close to the level that he was on during The HOTM arc, and add to it that he was still capable of growing stronger than he was during HOTM, because he has no upper limit to his strength, and it becomes plausible that even 8th Day Juggernaut would be defeated by a superior power to his own.
4. When Thor fought 8th Day Juggernaut, he momentarily made Juggernaut pause from one of his blows, this shows us that the Juggernaut of 8th Day could indeed be overcome by sufficient power. The Hulk of HOTM would be that sufficient power.