Why was the epithet varAha (वराह) used for Rudra in Rig Veda I.114.5?

Question

Rig Veda I.114.5 (dedicated to Rudra) says

दि॒वो व॑रा॒हम॑रु॒षं क॑प॒र्दिनं॑ त्वे॒षं रू॒पं नम॑सा॒ नि ह्व॑यामहे । हस्ते॒ बिभ्र॑द् भेष॒जा वार्या॑णि॒ शर्म॒ वर्म॑ च्छ॒र्दिर॒स्मभ्यं॑ यंसत् ॥५॥

We call down with reverence the boar of heaven, flame-red, with braided hair, turbulent in form. Bearing in his hand desirable healing remedies, he will extend shelter, covering, and protection to us.

In Puranas it was used to indicate the incarnation of Vishnu.

My question is - Why was the epithet varAha (वराह) used for Rudra in Rig Veda I.114.5?

Edit 31-8-2020

In the mantra व॑रा॒हम॑रु॒षं Can be split into varAham + aruSa . aruSa indicates shape of a varAha.

Answer 1

Varaha is interpreted as strength of a boar in this verse by the commentator Sayana. Rudra does not become a Varaha but is as strong as a Varaha.

Sayana: firm limbs like a Varaha.

Rig Veda Samhita 1.114.5, Bengali translation by Nilanjana Sikdardutta, Ramakrishna Mission Institute of Culture

Answer 2

In Puranas it was used to indicate the incarnation of Vishnu.

Not only in the Puranas but in the Vedas too Varaha is mentioned as an incarnation of Vishnu.

From the Mahanarayana Upanishad of the Krishna Yajur Veda:

uddhrtAsi varAheNa krishNena shatabAhunA

You (mother Earth) were lifted up by Krishna (Vishnu) with his hundred arms in the form of Varaha.

So this is not a "puranic concept" as you might think.

As for your main question, the translation you provided says this:

We call down with reverence the boar of heaven...

This just looks like a metaphorical description of Shiva as the might of heaven, comparing him to the might of a boar.