|Pet Battle Auto Forfeit, my personal choice amongst add-ons for this purpose|
- Your pets were obviously and catastrophically inappropriate for the battle. This is moderately understandable, as it is an account-wide system, so you can enter a level 25 pet battle zone and forget that you had your level 6 pets on your active roster.
- To get an advantage over NPC tamers. NPC's cannot alter their rosters - indeed, you are free and encouraged to catch and level pets to counter their teams - but they often have some variability in which pet they start. If they go out of the order you expected, you can either continue knowing your counters won't be ideal, sacrifice a turn and allow your foe a free hit on your incoming pet as you swap in the correct pet, or just forfeit.
- Because the wild pet/group you attacked does not contain anything you wish to tame. You might still want to kill the pets to farm them for exp, but this will take time you could be spending looking for other pets to tame. This being World of Warcraft, with its sometimes notoriously open scripting system, players have naturally coded UI add-ons to automate the process of determining whether the enemy team contains anything worth capturing.
Better Design As Intended?
As with most places where players are acting in ways the developer did not intend, some responsibility for the situation can be blamed on the game design. The NPC battle teams are tough - often higher quality than anything the player can obtain - and having to allow them a free hit on your group can be the cause of failure.
As to grinding for wild pets, well, here's where there is good and bad. Pets come in qualities - grey, white, green and blue - and, for those who really care about how those stats are distributed, in specific breeds as well. Setting aside the question of breeds, personally I won't spend time leveling anything that isn't blue quality, and you're going to be battling many copies of each pet to meet this goal - some estimates run around a 1 in 12 chance of encountering a rare pet, and some of these pets are somewhat hard to find in the first place.
If you know there is no way the pets you're fighting are the pets you want, and there's a good chance that someone else is going to come along and grab the other spawns while you spend the time on the fight, all of your incentives are in favor of throwing in the towel. (Ironically, world pet spawns are about as heavily contested as non-instanced content gets in a game that doesn't come anywhere near this level of competition for anything else in the open world.)
The changes may not solve the whole problem. In an additional tweak, any pets that people do run away from will respawn in the world so that others can try their luck. Thus, more considerate tamers can leave poor-quality pets in the world for those who just want to catch a copy of a pet they never intend to use, just to fill out their journals. In principle, though, this could open the door to forfeiting, quickly healing your pets from all the forfeiting, and re-engaging the same pet at full health.
In any case, I can definitely see why the way the system was designed led to the epidemic of forfeiting. Until the patch closes the door for good I'll be hard at work filling out as many slots in my journal as possible with the coveted rare pets (especially for any creatures I actually plan to use in combat).