I would say the majority of studies that have been conducted into this question, certainly those in Canada, indicate as you suggest, that the introduction of mandatory voting does reduce the number of applications for certification, their success rate, and ultimately the union density rate in the jurisdictions combined.
As for why that is, there are various theories, one of which you've just stated. In the context of an election, because the election takes a certain amount of time, there's always the scope for employer interference or attempts to persuade employees to vote against. There are competing theories that votes reveal true preferences.
None of the studies I'm aware of are able to determine which of those reasons is the true one, or whether indeed, as is most likely the case, it's a mix of the two.