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

Several MPs have claimed that the snap election has been called to prevent a looming expenses scandal. It has been reported that the UK’s Crown Prosecution Service is considering charges against about 30 Conservative ministers for breaching spending limits when it came to the 2015 general election. Any charges, it is thought, would have to be brought before the new general election, which is on the 8th June. This would give the CPS very little time to prepare the case against the ministers. By setting the date so early and doing this so suddenly, even though May has previously said that she wouldn’t, she is hoping that the case cannot be brought due to lack of time. However, the CPS has said that this is not necessarily the case, and that the cut off time for prosecutions varies. The specific allegations relate to Conservative activists who were sent to key seats, and whose expenses were included among a national campaign spend when they shoudl have been reported separately.