Why we say "расторг" but "моргнул"

Question

There are verbs with "-гнуть" ending, for example, ввергнуть, расторгнуть, моргнуть.

First two examples have following form in past time: вверг, расторг, but for "моргнуть" we have моргнул.

Can one give a reasonable explanation of the fact that this verbs are conjugated differently?

UPD: It worth to mention in this question that each of these verbs make up so called aspectual pair with imperfect form: ввергать, расторгать, моргать. And, actually, forms ввергнул, расторгнул are completely valid, as well as shortened forms. But this does not give me a hint )

Answer 1

There is a study which states that Proto-Slavic has two different suffixes -нѫти: one used for semelfactives (quanta of repetitive actions, expressed by such called multiplicatives), another one used for inchoatives and terminatives (verbs meaning "entering or leaving a continuous state").

Traditionally, in Russian they are denoted simply as perfective / imperfective aspects, but Plungian's theory states that these categories should be further extended.

Examples of multiplicatives (repetitive actions) and corresponding semelfactives (quanta of these actions):

• стучать / стукнуть
• кричать / крикнуть
• чихать / чихнуть
• моргать / моргнуть

Examples on continuatives ("being in a state") and corresponding inchoatives / terminatives ("entering / leaving a state"):

• замолкать / замолкнуть
• замерзать / замёрзнуть
• погибать / погибнуть

The study states that (though with numerous exceptions) semelfactives retain the suffix and inchoatives / terminatives lose it.

An interesting example would be the verb дрогнуть which has both meanings.

As a semelfactive, it retains the suffix:

Звездолет дрогнул несколько раз, точно успокаивающийся зверь, и замер. [И. А. Ефремов. Час быка (1968-1969)]

As a continuative, it loses it:

Видно, кто в этой шкуре не бывал, на морозе не дрог, тот нашего брата постигнуть и понять не может… потому ― душа зачерствела… [С. П. Подъячев. Мытарства (1903)]