I haven't heard of trees falling on the tracks very often, but this time of year trains are often disrupted for leaves on the line (I wonder if that may be what happened rather than trees falling on the tracks near Slough?)
Jules was using the route from Waterloo, but had to switch to the Paddington route.
But the same thing can happen the other way round in that the Paddington route is blocked for all sorts of reasons. When that happens anyone who has travelled on the cheaper Paddington route has temporary interavailability of tickets- that is the usual routeing rules are suspended. That only applies with physical or e tickets, not where you are paying by contactless.
Leaves on the line and trees falling are two sides of the same equation. At the time there was a lot of rain (the first decent rain in months) which is likely to have destabilised the earth on steep slopes, especially in railway cuttings leading to trees falling. Network Rail are doing a lot of work round the country to remove trees from beside the line, but it is a no win situation as that leads to it's own complaints of a barren landscape.
This would have been way more serious for homebound commuters on the Waterloo route, as they don't have the same options to re-route via Paddington. But then the locals know about the many local bus routes which serve that line, such as the 8 and the 10 to Staines (on the main line to Waterloo).