The consensus rule was pre-programmed to automatically activate after 995 of the last 1000 blocks (in theory 99.5% of the network) included a flag in the block indicating that they would auto-activate the new consensus rule. So think of it like green and red lights. If there is no set of 1000 blocks where at least 995 of them are green, then the old ordering is still valid.
Important to note that the new ordering is a stricter subset of the old, so new ordering is perfectly fine under the old rules, just old ordering is not acceptable under the new rules. This is called a SOFT fork. As soon as there are any 1000 blocks where 995 are green, then every block from that point onward MUST be properly ordered. (BTW, if you change rules such that something new you are doing would be invalid under the old code, like say, increasing the block size limit, it is called a HARD fork. Hard forks are considered much more disruptive to the network, soft forks are supposed to be easier to orchestrate)
By "showing green" the miners were supposed to be advertising that they would enforce the rule once the cutoff point was met. The problem is enough miners showed green, the cutoff was hit, but then when an "incorrectly" ordered block came up sometime after the threshold was met, too many miners still accepted the block as VALID and so now we have a significant (majority?) hashpower running on blocks which are "supposed" to be invalid.
Of course consensus is, by definition, whatever the majority is doing. So now it's a battle of dev's trying to cajole enough hashpower into enforcing the rule, or somehow backing off the change. With every new block mined on each head, there's more money at stake for which way the decision ultimately falls. For Bitcoin to survive, "There Can Be Only One".
It will be interesting to see how exactly this came about. Were there bugs in the trigger code which was supposed to activate the new consensus rule? Did ignoramus miners simply turn on the flag without actually priming the new consensus code for activation? Did miners mistakenly think they just needed to show "green" and then their blocks would be valid, but then they published a "green" block with the older ordering?