As usual, will answer your question in parts :-
I just want to know on what basis the upvotes are being reversed?
Reason is already provided to you by the system, they are reversed because of Serial Upvoting
I have also voted many posts in a day. Will this count as serial voting?
No, this won't be counted as a serial voting, but if there is anything suspicious, say, you are voting a specific user only then surely they will be reversed manually if not in 24 hours then after certain days (if you escape from system auto reversal).
I think whoever has upvoted my posts had read them and upvoted me.
No he didn't and hence system reversed his bulk voting. If you see your profiles reputation tab, I'll explain your first reversal here which is -140
Over here, system has reversed the votes which were cast by the user in a minute
But if you see, system didn't reversed the +10 reputation which was legit. Now if you say that the user read all your post in a minute is obviously not possible hence, this justifies first reversal.
Coming to the second reversal which was also for -140, where the user votes your posts in a minute again
So over here, system reversed entire reputation because the user cast the votes in a minute. On the other hand, you do have high upvotes but you up vote good posts regardless of the user and hence your votes are distributed evenly. Also, you do not vote the posts in bulk.
It is just he/she might have visited my profile and started to look at my each post, which I think is normal.
It is normal, but voting without reading the posts isn't.
Neither I have relation with the upvoter nor I bribed them to vote me
Yes you don't and you didn't.
So what should I do now?
Don't worry, I've checked your profile and everything's clean at your end, if this repeats for the third time, the user who is behind this will be banned but you need to flag a moderator to notify this behavior if happens again.
Note : Serial voting is taken seriously on StackExchange, any foul
play with any user will result in suspension, so don't play with your
privileges, use them wisely.