AsWhile it might be that the 'bumping' causes subtle delays in transforming when landing, you seem to have controlled for, often the reason that a group of Vikings won't finish transforming at the same time is because they're colliding with each other and/or can't find a valid space to land if it's too crowded. The latter An only tangentally-related issue is readily apparent ifwhen you ball them all up so they're stacked then trycommand several to land them all at the same time;a time in a small space, is that some will just totally ignore the commandwon't at all because it's too crowded.
The nominal transform duration is 3 (game)game seconds, which is about 2.2 real seconds on "Fastest", however there is a small degree of randomnessvariance beyond just the animation. This is readily apparent if you give the vikings a move command while transforming, into either air or land mode as some will set off before others. Additionally, it doesn't seem deterministic as far as where the unit's selected in a control group, thegroup—the first to transform one time won't always be the nextnext—so it is possibly random.