As the other answers have mentioned, in classical Japanese サ変's 未然形 was せ and thus the passive せられる is just the standard 未然形 + られる. Some of these super irregular する verbs still retain this characteristic. However, some of these irregular words have changed and are more similar to 五段 or 一段 verbs now. For example, 愛する conjugates into 愛さない, behaving similarly to a す-ending 五段 verb for negation.

Here's a summary of part of [サ変動詞の活用のゆれについて--電子資料に基づく分析][1], which is based on a digital textual analysis of Asahi Shimbun from 1987-1992. It discusses this at length:

The author categorizes する verbs into 4 classes, but I'll stick to talking about the second class which he calls "Unstable words in between *Sahen* and *Sa-Go* [属する type]" (`サ変とサ五のあいだでゆれているもの一「属する」類`).

These are the words that he lists:
>（1）愛圧逸臆科課介[解]{かい}害画冠関期帰記擬議喫窮御供遇屈[解]{げ}激決抗刻察死資持辞失謝熟処叙称証詔賞食制接絶宣奏即属存堕対題託達脱徴呈適徹毒鈍熱廃排配縛発罰反比秘表評貧付復服偏滅面模目訳有要擁浴利律略類列労和, etc

Then, using textual analysis he postulates the following:

> （i）「X」が促音・撥音・長音を含む場合はサ変のままであり (words with 促音・撥音・長音 conjugate more similarly to サ変)

> （ii）それ以外の場合はサ五に変化している。 (otherwise conjugates more like す-ending 五段 verbs)

For many conjugations, this heuristic works out pretty well and is probably pretty useful for figuring out how to conjugate:

96%+ for negation with ない, 82%+ for negation with ず (disregarding 期せず), 92+% for negation with ぬ, 75%+ for conditional with ば (disregarding 愛すれば).

However, for the passive it's only about 50% correct: 

> 「Xせられ」と「Xされ」の用例数の総計は，（i）において592対652（「Xせられ」が47.6%），（ii）において1,554対2,237（「Xされ」が59.0%）となる。

[![enter image description here][2]][2]

Judging from their analysis, I'd say 発される is pretty much never used and is nearly always 発せられる.


  [1]: https://doi.org/10.15084/00002053
  [2]: https://i.sstatic.net/6f3bB.png