Learn Spanish (Latin America): Se me descosió el bolsillo del pantalón ayer. - My pants pocket came unstitched yesterday.

This is the "accidental se" structure. It's used to emphasize that an event was unexpected, involuntary, or an accident. The se indicates the action just sort of happened on its own, and the indirect object pronoun me shows who was affected by it—you are the "victim" of the rogue pocket!

Why is the verb descosió conjugated in the third-person singular?

In the accidental se construction, the person affected (me) is not the grammatical subject. The subject is the thing the accident happened to—in this case, el bolsillo (the pocket). Since el bolsillo is singular, the verb takes the singular form descosió. If both pockets had come unstitched, it would be se me descosieron los bolsillos.

English uses the plural "pants". Why does this sentence use the singular pantalón?

In Spanish, especially in Latin America, it is very common to use the singular el pantalón to refer to a single pair of pants. You will also hear the plural los pantalones, but both are perfectly acceptable for one garment.

I see the verb coser hidden in descosió. How does the prefix change the meaning?

You spotted the root! Coser means "to sew". Adding the prefix des- reverses the action, functioning exactly like the prefix "un-" does in English. Therefore, descoser means "to unstitch" or "to unsew".