In his post of 27 November 2008, John Wells mentions the substitution of ‘-uous’, ‘-ual’ for ‘-ious’ and ‘-ial’ in the pronunciation of these two words. There is a more widespread confusion in the minds of English speakers about words ending in -ious, -ial, -iary, -uous, -ual and -uary (and probably -iate and -uate, although I have no examples to hand).
I have recently heard judiciary pronounced /dʒʊˈdɪʃʊəri/, sumptuous as /ˈsʌmpʃəs/ and sexual as /ˈseksjəl/.
I think the problem lies in the pronunciation of the -u- in those words that have it in the spelling: /jʊ/. The yod tends to palatalize the preceding fricative (/s/ > /ʃ/) or plosive (/t/ > /tʃ/ and at the same time to front the following vowel, so that /ʊ/ moves towards [ʉ/ or [ɨ]. This leads to the only remaining distinction being between the /ʃ/ and the /tʃ/. John says that the OED pronunciation given for rumbustious, ending /-tiəs/ “sounds very prissy”, and I agree with him. However, celestial, with an ending in the same category, does not sound at all prissy to me when pronounced /səˈlestiəl/.
Narrowing down the endings to those with -ti-, the regular treatment is to pronounce the two letters together as /ʃ/. This occurs in almost all the words ending -tion, for example (there is one exception where the fricative is always voiced: equation /ɪˈkweɪʒən/, and one partial exception: transition, which many people pronounce with a voiced fricative: /trɑːnˈsɪʒən/ – I think this would be the pronunciation of transcision, but then I’m not an RP speaker). There’s also consortium, which is usually pronounced /kənˈsɔːtɪəm/, but which Leon Brittan (British Home Secretary at the time of the Westland scandal in the mid-1980s) notably pronounced /kənˈsɔːʃəm/. The plural, consortia, is more often pronounced with the /ʃ/ than is the singular, although this is still probably a minority pronunciation. Nasturtium (garden plant) and inertia regularly have the fricative /ʃ/.
Going back to rumbustious and nuptial, the first “needs” the /t/ to be retained, or we end up with /s/ followed immediately by /ʃ/, which would turn into /rʌmˈbʌʃəs/. It is then being compared with, and pronounced by analogy with, tempestuous and contemptuous. Nuptial is presumably being pronounced by analogy with virtual and mutual. The preceding /p/ is not a factor, since we have captious, which at the moment at least, is not becoming “captuous”.