There's been a significant rise in the number of applicants these past few years, and not just at Imperial but it seems for all computing courses.
Cambridge now receives 4x as many computer science applications as they did in the 2011/12 cycle. UCL receives more than 3x as many as it used to. At Imperial the number of applications has nearly doubled in five years. This is all for BSc/BEng. But they can't start taking in 3 or 4 times as many students as before, they're still limited by the cost of teaching, the space available, the staff available. So the result is it just becomes more competitive.