For reasons I hope are obvious, (1) these "simple checks, heuristics or algorithms" are in place and (2) their details--even their nature--are not publicized.
When you think you have been a victim of "revenge" or "serial" downvoting, please do not post the usual "why the downvote?" comments. Instead, flag one of the downvoted posts and use the "Custom" reason to explain your concern. A moderator will look into it.
Note that moderators do not have any powers to change votes. We do have the ability to contact users privately about such voting (there is a standard e-mail template for this behavior--it's not uncommon, alas). We can also contact the SE team directly with a request to look into such things. The team always responds, but it can take a few days. Any high-rep user also has the ability to compensate for untoward downvotes by (1) adding their own upvote (if they hadn't done so already) and (2) offering a bonus to make up for the lost reputation and to provide explicit public support for a particular answer.
In this particular case (rather than waiting for the automatic mechanisms to kick in) I have confirmed these downvotes and learned who made them; that they are the first and only downvotes made by one user; and I have contacted the SE team to investigate it.