I assume "за сосисками" translates to "for hot dogs". But if за means "for", shouldn't сосиска be in the plural accusative case, not instrumental case?
The preposition за has several meanings, and according to its meaning it can be used either with the Accusative case or with the Instrumental case.
In your sentence за means the reason or the object why you go somewhere, the object you plan to take or buy somewhere. In this meaning, за is followed with the Instrumental case.
Он пошёл в магазин за хлебом. - He went to the store to buy some bread.
Муж поехал в город за новым платьем для жены. - The husband went to the city to get/buy a new dress for his wife.
Ему пришлось вернуться домой за ключами от машины. - He had to go back home to take his car keys.
Сходи в детский садик за ребёнком! - Go to the kindergarten and take back our kid.
Полиция следила за ним. - The police were following him.
To be completely honest with you there's not that much sense in your question. It's like to ask why particular phrasal verb in English contains exactly this combination of words. Why we are saying "write it down" instead of "write it on" - well, just because this is how it happened in language.
It is ruled by "стоять в очереди за чем?" - and this is the way it is. No other option rather than just to memoize.
Most languages that use prepositions tend to have a handful of core prepositions do a terrific number of different jobs — too many to reliably predict their behaviour in another language even when the analogy seems close and clear enough.
The за-accusative and the за-instrumental may both more or less translate into English as for, but they're nonetheless distinct. Accusative is used when it's for as in: "instead of"; "in support of"; "for a price/fee of"; or, "in reward/punishment for". For can also mean "in order to acquire", and also translate as за in this sense, but Russian treats it as not quite the same за and gives it the instrumental.
"Instrumental case" is not quite right translation of "творительный падеж". Most correct translation would be "ablativ".
Mostly we use this case as instrumental case, but it also has around 13 more functions. Full list of them you can find in Russian wikipedia.
In your sentence "творительный падеж" is used, because preposition "за" mostly indicates place, and place is one of the functions of "творительный падеж".