Why is one not required to pay if one causes damage on Purim?
Does it apply in all situations?
The question is addressed in the poskim, I believe it gets a footnote in Shulchan Aruch at the end of the Laws of Purim. I recall hearing a tape about this a few years ago.
In short, drunkenness alone is not a defense (see below); what may be a defense is that if the damages were caused "as part of normally-acceptable merrymaking." Tosfos (France, 1200s) talks about weddings where the norms involved young men jousting; does everyone at the wedding imply they're entering at their own risk. Similar discussions occur with regards to someone getting poked by a Lulav when everyone is carrying them around. I recall another responsum about sheva-brachos dinner celebrations at which it was the norm to toast the couple, then slam your glass against the wall. (Well eventually someone lost an eye because of this ... don't ask me who started these ideas; it reminds me of a Dilbert strip which observes how weird of a habit smoking is; "would you like the slap-yourself-with-a-sea-bass section of this restaurant?")
The conclusion appears to be that "small" damages are waived if incurred during normal merrymaking; larger damages are not. With the major caveat that the Aruch HaShulchan has none of it, damages are damages.
So the fact that it's Purim only matters with respect to what's considered normal merrymaking activity.
As for drunkenness: a drunk driver who kills someone is, in my humble opinion, analogous to the Talmud's case of a man walking around on a pitch-black night with an outstretched dagger; if someone is killed, you don't call that an "accident"; you call that "gross negligence very close to murder."
Otherwise, halacha has two settings for drunkenness: "conscious of one's actions", and "not conscious" [higiya leshichruto shel lot]. A person can be seriously tipsy and impaired, but so long s/he is conscious of his/her actions, full responsibility (civil and criminal) of their actions would be enforceable. A person intoxicated past the point of consciousness is in the same halachic category as someone completely insane or having an epileptic attack (see Rambam's commentary to Gittin 7:1). Such a person can not be charged for criminal actions; however, with regards to monetary damage, adam mu'ad le'olam; if I choose to go to sleep right next to someone's expensive camera, then roll over in my sleep and break it, I'm responsible to pay for consciously putting myself in that situation. So too, there is absolutely zero mitzva on Purim to drink to this severe point, thus anyone choosing so bears full monetary responsibility for their actions.
The above paragraph is from a major Achron ... I believe it's the Shach ... concluding, "and anyone who's ever studied for one day will see this is correct."
A lesser-known midrash on Proverbs warns on the dangers of drinking: "and if one strikes his fellow when severely drunk, he will not have to pay, which makes it impossible to atone for such an action." Rabbi Chaim Kanievsky sees this as proof that the severe drunkard is let off the hook; the more conventional interpretation would be either: a.) this is in the rare case where the person became severe drunk not due to their own decision (someone spiked their drink) b.) the strike incurs pain and embarrassment (such as a slap on the face), but not lasting monetary damage; the former are only charged as a result of intentional action.
If the damage was unintentional, then he is exempt if he caused minor damage. from payment - Rama 695:2, Matei Moshe 1013, see Bais Yosef 695. Refer to Meseches Succah 45a, Rashi “miyad, Tosfas “miyad,” Shulchan Aruch C.M. 378:9. See Pri Megadim Eishel Avraham 7. Mishnah Berurah 695:13, see Refer to Bach (yeshonos) 62.
There is an opinion in the poskim that exempts him even for physical damage, while others are stringent. Magen Avraham 7. Refer to Bach (yeshonos) 62. Aruch Hashulchan 10, see Elya Rabbah 15.
The definition of damaging due to joy is unclear. In any case, the Aruch Hashulchan says that in his days the joy we have for Purim is not the type of joy which would exempt one from paying for damage. - 695:10
In addition, one who grabs food or drink from his friend on Purim is not considered stealing. Rama 696:8.
Here too, the Aruch Hashulchan says that this leniency is not applicable today. - 696:12. See Toras HaYeshiva 13:14, Chazzon Ovadia Purim page 208
Source: Halachically Speaking 10:3 page 17 (pdf)