Leaf fall, surely?
The printed timetable says:
"Travelling from 10 October?
Train services will be amended due to seasonal conditions. Please check carefully before travelling."
That would seem to chime with when I saw the change.
From what I can work out the extra 2 minutes seems generally to be Mondays to Fridays in the down direction between Reading and Guildford then it runs as "normal" after that.