The non-BJP parties like Congress and the Mahagathbandhan are not willing to accept their defeat in the general elections of 2019.
What reasons do these parties state for not accepting the election results?
So firstly, the election result hasn't been announced yet. The result is expected on Thursday 23rd May, at the moment all we have are exit polls and several high profile elections/votes recently make clear these are not fully reliable.
Vote Machine Tampering, that said it does look like several parties in India are positioning themselves to suggest that the election was not honest. Allegations of vote tampering are very serious and the current position taken by opposition parties is that there is not a documented procedure for responding to mismatches between the Electronic count and the paper verification.
According to the linked article;
The Supreme Court has ordered the EC to tally the results from five EVMs with VVPAT receipts in at least five polling stations in every assembly seat. A parliamentary constituency comprises several assembly seats.
The Supreme Court is attempting to make sure the electronic count is being checked on a sample basis. It is what happens if an error is found that appears to be the basis of the current dispute. Opposition parties want the entire seat to be counted via the paper ballots if there is a single discrepancy. There are 543 constituency seats made up from 4120 assembly seats and 1.035 million polling stations in this election. The sampling would cover at least 25,000 stations, possibly many more . You'd need to consult a statistician to see if this was a valid sample size for detecting possible errors.
According to Wikipedia, while EVMs have been used in India since 1998, the auditing checks were only introduced on a pilot basis in 2014 and this election in 2019 will be the first one to use the VVPAT (Voter Verified Paper Audit Trail) for the whole of the vote. As such the procedures for dealing with discrepancies will be hugely important as they will set the standard for many years to come.
"On VVPATs and the EVM tally, the EC is yet to come out with a procedure in case there is a mismatch. Even if there is one mismatch in the EVMs or VVPAT samples picked for counting, to maintain the integrity of the electoral process, all VVPATs in that Assembly segment must be counted. This is important to maintain integrity of the electoral process," Mr Yechury said.
Rather than rejecting the result, which isn't actually out yet, it appears the opposition parties are interested in ensuring the count is accurate and are distrustful of the EVM process.