What has motivated the UK's Prime Minister Theresa May to call a snap general election?

In the 2015 election, the Conservatives were victorious. Over time it became apparent that the party used underhand tactics in many of the areas where they were at risk of losing a representative. In those constitutencies, special attention was given to ensuring the result the party wanted, and young Tory activists were brought in to boost support for their party. Election rules specify that a party can only spend £15,000 campaigning in a given constituency. The money spent on transport, hotels, and entertainment for the activists took the sums involved higher than £15,000 and the Conservative Party did its best to camouflage that spending. It's feasible that the 2015 election was won by the Conservatives directly because of the illegal spending on the activities of young campaigners. The party has already received a record fine for the offence, and the decision to prosecute those involved must be made no later than early June. That provided Theresa May with a powerful incentive to call an election that would take place before the ensuing court cases, which could involve 29 Conservative MPs being put on trial for fraud. The potential damage to the party's reputation would be enough for it to quite possibly lose the next election were the results of those court cases be known beforehand.