For the correct answer, Easyjet would need to supply the computer algorithm it uses.
But I doubt that it is simply a matter of the price rising as the seats are sold.
A much more likely scenario would run thus:-
Imagine that there are 200 seats on the flight, and on average they sell one seat a day from 200 days out. Clearly if there are less seats available than days until the flight, the price is likely to rise, regardless of the number of seats remaining.
I have long suspected that airlines monitor those checking the availability of seats, and regardless of whether a booking is made as a result of the enquiry, the price increases due to someone showing an interest in the seat. So you regularly checking the price of a flight could be influencing the price (in the wrong direction).