In Dawn of War II, if you capture the entire map, you deny your opponent resources, and they can't even build INFANTRY, let alone armor.
In CoH2, EVERY SINGLE MATCH is just the opponent sitting back, losing the ENTIRE map, then building two or three tanks and winning without any actual tactics and without being punished for failing miserably at holding anything early on.
Why is this?
Why is a player who loses both fuel points, both munitions points, and the vast majority of control points allowed to build any kind of threatening units at all, let alone several of them?
Why does capturing said resource and control points not actually accomplish anything in terms of harming the opposing player?
What is even the point of capturing territory, when it has absolutely no effect on anything?
The announcer shouts "The enemy has cut off our supply line!" in an exaggerated tone, as if there's real danger in being cut off, but in reality, there isn't...
And was it always this way? Or did CoH2 start out like DoWII?
Like others said, it's a comeback mechanic for the losing players to give them a chance. That and the manpower income is very high when said losing side barely has anything.
As for the sudden appearance of tanks, I have a few theories since you haven't presented a replay:
1. Player was hiding his tanks. (Very unlikely)
2. Had an ally that provided fuel for said player.*
3. Players was saving his fuel for something big.**
*Some commanders have the ability to call in fuel drops for anyone to use, but since you seem to have mentioned that the losing player(s) was supposedly pushed back such abilities would have a role in allowing players to obtain fuel.
**This is the most reasonable one since my theory for your match was that your opponent was saving up for a heavy tank. However, because you were winning he needed an immediate response to counter your forces. Rather than spend precious time getting a heavy tank, he chose to make multiple mediums in quick response.
While I could make more theories, it's better if you post a replay. Saves us plenty of time and guys can have a look at what happened.