Hi
First of all, This sentence is a statement. it falls into pattern 8 in Relation Grammar ( dummy subject+Linking verb+noun phrase/subject complement.(Let's Write English, Wishon, V I, Basic Patterns). This part of the sentence is called the ' pattern ' and the indespensible part of the structure. It can even stand alone structurally though the richness of the message will go away ( It was the White Rabbit.), but the rest of the stucture is what is called modifier which is mainly used to limit the scope of the sentence ( the term 'modifier' is used in Relation Grammar and in linguistics ( syntax ))
to understand the structure we need to decomplex it in this fashion:
It was
the White Rabbit.
He was trotting slowly back again.
He was looking anxiously about. [we can guess]he had lost something.
This structure is rip for noun-modifier complexing since 'The white rabbit" is the antecedent of 'he',so:
It was the white rabbit who was trotting back again. He was looking anxiously about...
AND
It was the White Rabbit who was trotting back again,
looking anxiously about. The highlighted PHRASE is called verb modifying participial pharase (Modern English, Marcella Frank , VII, Participial phrase) and it defines how the verb "trotting back" was being done.
The adjective clause ( who was trotting back again) is a noun modifier which limits the scope of white rabbit and here in particular it is a Non-restrictive Adjective Clause since the noun itself is limited because of its being a proper noun. An adjective clause with a subjective subordinate conjunction can be turned into a participial phrase/ noun modifier ( Losing 'who' and 'was') without distorting the meaning and the message, so:
It was the White Rabbit, trotting slowly back again, looking anxiously about, as if he had lost something.
The comma in which the participial phrase is enveloped is due to Non-restrictiveness of the adjective clause. ( ME II, Marcella Frank, Adjective Clauses.) Now the analysis of the structure is :
It was the White Rabbit[Main Pattern], trotting slowly back again[participle PHZ/Nm], looking anxiously about[participle PHZ/Verb modifier of Manner].
as if he had lost something. Now what is this?
This is what is syntacticaly called " Adverb clause of manner ( speculative-subjunctive). "speculative" because we are describing his "looking anxiously about" through guess work and since the guesswork is not so firm it is referred to as subjunctive. Another reason for Subjunctivity is our recognizing it as a remote possibility. It is interesting to know that it can also be turned into what we call as "elliptical adverb clause of manner, given the Clause subject and the subject of the pattern (matrix) are the same;hence:
It was the White Rabbit, trotting slowly back again, looking anxiously about, as if having lost something.
The referring to Verb Modifier and Noun Modifier is a common mistake that happens in non-specialized grammar circles. VM and NM are Relation grammar concepts; Adverb and Adjective are Parts-of-speech grammar jargons. An adverb, apart from a variety of types ( place,time, manner,frequency,viewpoint,and intensification) can have the following functions:
He was crying silently. ( 'silently' here is adverb of manner/ verb modifier)
I was spying the silently indignant child. ( 'silently' here is adverb of manner/ adjective modifier) since it restricts the scope of the adjective 'indignant' .
Brightly resplendant diamond. surrupticiously friendly guy.
ADVERBS are modifiers with the freedom to appear at the beginning, in the middle, or at the end of patterns and it doesn't reaaly matter how many you can cram into a structure. As long as the restricting functions are not vagure and the referrences are clear, you are all right.
'Back"' is the Adverb of direction/place (MEI, adverbs)
'again' is the adverb of frequency. both are verb modifiers, modifiying the verbal ( participle) 'looking'. To convince you, I can add even a greater number of adverbs:
Unbelievably, It was the White Rabbit yesterday, silently trotting slowly back again, looking anxiously and suspiciously about, as if he had unfortunately lost something again.
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Grammar Analysis: Pls help! 
Linear Mode
