@Jelly: "Accidental" here means that you have no control over these things that happen. You might find useful
this discussion about the uses of "se".
In addition:
"Le está saliendo agua de la mochila" -> The speaker could have used pronominal particles, but it seems he/she preferred an indirect object pronoun (le) to include the person carrying the bag that was leaking water.
With a pronominal particle, the speaker would have used a different structure: "se le está saliendo el agua de la mochila" / "se le está saliendo agua a la mochila", and here it's only the bag involved, not the carrier.
"Te arrastra el vestido" and "se te arrastra el vestido" are expressing the same idea. Many people tend to avoid the emphasis/accident function of "se", because they feel it's colloquial; for me though it's not a matter of correctness, but of personal preference.
"No me gusta cuando el agua
me entra en los ojos" -> There is no "se" here, because the pronoun (me) is an indirect object one.