What did I do wrong - recharging sealed lead-acid battery with recycled computer PSU
When you fill the battery, it will start showing a voltage across its poles; this voltage will push a current into your PSU and power parts of it. Most PSUs don't like this very much.
If you connect things wrong, a large current may flow into the PSU and fry it.
Also, the PSU will generate a voltage of 12V, and will only be able to charge a (nominally) 12V battery that has been discharged to below 12V. When charging, the battery's voltage will (try to) rise very quickly to more than 12V, and the charging will stop. At this point, very little charge will have gone into the battery and it will only be charged partially.
So, a 12V PSU is not suited for charging (nominally) 12V lead-acid batteries. These batteries should be charged with a current source until their voltage reached about 13 to 14V, see battery specification.