This trend is not isolated to Infosys alone. In fact, most employees at a given company don't stay beyond 2-3 years as they seek new pastures where they can develop their career further. This trend is most observable amongst young professionals who are more willing to seek change rather than their older colleagues who prefer to settle down at one company eventually.
The reasons to leave a company can be many. Some engineers look to pursue post-graduation with options such as MBA and MS on the table. Some on the other hand prefer moving to a new company setup with a much higher designation than the one they previously occupied. Some even leave their jobs to go on a year's sabbatical to get a breather from the hectic way nature of life.