The first question of yours.
What is the purpose of bootstrap customize?
That seems a bit off-topic, why? Because it is not generally related to coding. You're not showing any code, any search results that you have while you searched for answers. So a question that is off-topic gets downvotes. Or should I say, heavy downvotes. But that is not meant for off-topics only it is for duplicates and all other kind of questions too.
how to make input fields editable?
In this question, you're a bit puzzling. However this question must not be downvoted.
how to pass the checked values of checkbox
But this question is just the same (or lets say, somehow similar) to the last question. That is why, this one is getting a downvote.
Coming to your points, this much fast voting can be casted by a single user. Three events that happened between
21:39 - 21:40 are more likely to be an articraft of a single user should I say. However, you might want to wait the deleted post with downvotes will generate rep soon (well, it generates rep for me once moderator deletes a post with downvotes after a few minutes).
You have tagged a question for
serial voting but have you read the answer? The answer is clear that the serial voting is meant to be voting on many post. These 3 posts won't be counted in many.
If that was a serial voting (down or up) system would have jumped into the fight and provided you with the rep that you have right on and would have punished the user doing so. Since system is well-aware of basic algorithms to conclude which one is a serial and which one is fair voting it took no action.
Edition to answer
I came to one more point for you. The server at SE keeps you network information too, using that system it will note when a user upvotes your questions or downvotes. By using this algorithm it will take action and do what it thinks should be done! So don't worry, this might be a co-incident by 2 - 3 users downvoting your questions which were in the active tab all at a time. Sorry about this, but this is also a condition here.
Try to read the basic posts about posting the questions and answers on SE
Links to read:
https://stackoverflow.com/help/on-topic (How to stay on topic)
https://stackoverflow.com/help/dont-ask (What should not be asked here)
https://stackoverflow.com/about (Go to the Get answers to ... section)