A verb glossed “fell” in the Lament of Akallabêth (SD/247), but it is clear from context that its meaning is different from the ordinary verb for “to fall”: kalab-. Perhaps the sense of dubud- is “to fall under the influence of [shadow]” or “to fall under the cover of [shadow]”. Between the second draft and the final version version of the poem, the verb form changed from dubbudam (past tense) to dubdam (aorist), in both cases with the plural verbal suffix -m.
Carl Hostetter and Patrick Wynne suggested (AAD/23) that dubud- may be related to the Elvish root ᴹ√DUB “loom, hang over oppressively”, which would fit well with the sense “to fall under the cover of”.
Conceptual Development: The draft form of the verb was udūbanim, which is difficult to analyze. It is missing a second d, so the draft verb may be a biconsonantal-verb dū̆b- or perhaps a triconsonantal-verb dūban-. Patrick Wynne and Carl Hostetter suggested (VSH/35-36) that the initial u- may be a 3rd-person pronominal prefix, but I think it may instead be the vowel augment of a draft-perfect tense; see that entry for further discussion.