Gfrktyl

Why Do Languages Change? (2010) by R. L. Trask. pp. 5-6

    Our story moves now to Scotland, where the word grammar underwent a
small change of pronunciation to glamour, reflecting the awkwardness of
having two instances of /r/ in one word.

Can somebody please help me sympathize? grammar isn't awkward for me to pronounce.

The only sensible interpretation of that claim that I can see is that having two instances of r in a word poses a special articulatory challenge. However there is no evidence to support that claim. A more plausible explanation is that the cause is perceptual. First, we may assume (there is some evidence) that r has a subtle long-distance effect in words, lowering of F3. In a word with two rhotics, it may be difficult to tell what the source of the general rhotic flavor of the word is, so listeners filter out one of the rhotics as being a phonetic side-effect of the other. This occurs in other languages including Georgian, Yimas and Sundanese.

Since there was a question in the comments about the existence of the phenomenon I have searched the Corpus of Late Modern English Texts for short words of the pattern C[rl]V+[rl]e? with the corpus query processor. The phenomenon clearly exists on a statistical level, with two l's close to each other being the most deprecated combo:

 Combo  tokens types  sample words 

 Cr_r     1856    25  prayer, prior

 Cl_l       16     1  flail

 Cr_l     4561    20  cruel, trial

 Cl_r     7931    35  clear, floor

The pattern for the search was [word="[b-dfgk-npqstvwxz]l[aeiouy]+re?"] (for the last query).

I don’t know if that helps you because I am German, but I find two r awkward mainly because to me it doesn’t sound like a legit syllable. In the German language, there is no letter that sounds like the English r, so to me it doesn’t even sound as if I am speaking at all, but rather as if I was imitating a roaring lion or a large toad. I also struggle with words that, unlike rural, have a consonant between the two r (like grapefruit). Sadly, we also use the word Grapefruit in Germany as there is no real translation (some people call it a Pampelmuse, but that’s technically a different fruit).
Also, again that probably doesn’t contribute 100% to your question as it refers to another language, but there are examples of words or names with two r that are changed or pronounced differently when they find its way into our language (like the character roronoa Zorro from the manga/anime One Piece, whose name is the same in English, but in German he is called lorenor zorro which has only one spoken r.) it also apart from the th the most difficult to pronounce for most English language learners and I‘ve often heard people pronouncing words like „rapper“ more like wapper (whatever that is). For a native English speaker, this barrier obviously isn’t there because they are used to it, but probably the „this r sounds like a toad coughing something up and two r make it sound totally awkward“ stays the same?

When there is a homorganicity phenomenon, there is generally a tendency to distinguish the phoneme from its twin (dissimilation) or to fusion them in a same phoneme (assimilation). That can occur even though those phonemes are far away from each other (like in Latin, see aris/alis). So, phonetic is not at stake, it is a phonological phenomenon.