Unfortunately this seems a fairly common sight in BtS, particularly on aggressive AI settings. It gets fairly predictable:
1)AI builds huge stack of early units, slowing its science rate and neglecting libraries etc to pay the upkeep.
2)AI attacks neighbour with huge stack of units - if neighbour is AI they respond with equivalent stack. If neighbour is human, they are either destroyed, or hole up in a highly defendable site and let the AI beat themselves to death tryng to take a fairly pointless hill/forest.
3)AI is falling behind research-wise, but builds a second stack of ancient age units - again slamming its research to 0%, and attacks neighbour
4)If human player survived 2) they probably have medieval units by now, and find defending fairly easy.
5)AI builds third stack, still stuck in the ancient classical era.
6)Human player has by now hit rifling, and even with vastly lower unit numbers has comparable power ratings to the AI.
7)AI now doesn't attack human, as it quite rightly realises that its obsolete units will get slaughtered. However it now sits there on 0% science with its economy bled dry by colossal numbers of useless units.
8)Human can now cruise to space race, or flatten the AIs macemen and longbowmen with tanks and aircraft.
Basically if the AI's first rush fails to beat the human player (at least on Aggressive AI), the human has already won. The same is true to a lesser extent on normal AI. The AI builds excessive numbers of outdated units, and has a very poor tech pace as a result.