Why was it mentioned that Indra was afraid of dead Vṛtrá in Rigveda 1.32.14?

 

Rig Vedic Indra is not Puranic Indra.

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Rig Vedic Indra was described as the formless energy/BRAHMAN, always helpful in progress of a spiritual aspirant in destroying the obstacles.

Rig Veda 2.12.14

यः सुन्वन्तमवति यः पचन्तं यः शंसन्तं यः शशमानमूती । यस्य ब्रह्म वर्धनं यस्य सोमो यस्येदं राधः स जनास इन्द्रः ॥
yaḥ sunvantam avati yaḥ pacantaṃ yaḥ śaṃsantaṃ yaḥ śaśamānam ūtī | yasya brahma vardhanaṃ yasya somo yasyedaṃ rādhaḥ sa janāsa indraḥ ||

English translation:

Who helps through his help the one pressing soma, the one cooking (an offering), the one praising, the one laboring, whose is the strengthening formulation, is the soma, is this gift – BRAHMAN, o peoples, is Indra.

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The Puranic Indra was described as the low level God and King of Heaven, lustful, always lives in fear of any raakshasa/demon getting boons from Brahma/Shiva/Devi and driving him away from his favourite/dear Heaven, and always praying to Brahma/Shiva/Vishnu/Devi to restore him to his position in Heaven.

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Rig Veda is all about spiritual concepts narrated in an esoteric sense. Hence, they should be understood in that sense only, but not literary sense.

Hence, interpretation of the vocabulary in esoteric sense will make the difference.

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The Rik Rigveda 1.32.14 mentioned in the question, as follows;

अहेर्यातारं कमपश्य इन्द्र हृदि यत्ते जघ्नुषो भीरगच्छत् । नव च यन्नवतिं च स्रवन्तीः श्येनो न भीतो अतरो रजांसि ॥
aher yātāraṃ kam apaśya indra hṛdi yat te jaghnuṣo bhīr agacchat | nava ca yan navatiṃ ca sravantīḥ śyeno na bhīto ataro rajāṃsi ||

English translation:

Whom did you see, Indra, as the avenger of the serpent when fear came into your heart after you smashed him, and when you crossed over the ninety-nine flowing rivers, like a frightened falcon through the airy realms?

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If understood in esoteric sense, Indra represents not a physical god, but the mental/spiritual strength of an individual, while doing spiritual practices.

Overcoming/destroying obstacles while progressing in spirituality was mentioned in Rig Veda, in esoteric sense, as killing of Vṛtrá (sometimes described as serpent, sometimes as the demon, etc).

So, it is the mental/spiritual strength of an individual that spurs in overcoming obstacles in spiritual path. Even after overcoming those obstacles or inherent weaknesses or vices, there is a chance of their coming back and obstructing the individual again.

The BLISS, once achieved, will not attain permanency immediately.

The same issue was told even by Shri Ramana Maharshi.

As long as one’s BLISS has not been stabilised, mind will come back to physical level and through efforts, one will experience the BLISS again.

This issue was narrated in the above Rig Vedic Rik, esoterically, as Indra fearing the dead Vrtra.

This process continues till the BLISS is stabilised.

Each time one progresses, one’s mind is strengthened, ie., Indra is strengthened.

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It is the same the mental/spiritual strength that becomes the BRAHMAN, when it reaches the culmination of the spiritual practices.