The Railways decision to close the railway because of an external incident / event is will out of the Railways hands.  It will be the local civic authorities (Police, Fire, Council) that put the exclusion area in place, the Railway Route Control would work with the requestor for the closure to minimise the impact.

Are the exclusion area getting larger? possibly, working on the bases of its easier to scale back than expand.

Response to events like gas leaks, the one yesterday at Didcot must have been a high pressure pipeline or in a distribution location.  I would think there is a whole procedure for making the pipeline / distribution equipment safe, it could be there are limited specilist teams to work on these that had to travel across the country

 
Are these third party issues becoming more prevalent?  Are we simply noticing them more?  Are we being more careful than we were before in shutting down as a precaution?  Are we taking longer to sort things out? Is the disruption more widespread because of the limited resources and increased specialisation that is the modern railway?   Is it just a bad morning and I happen to have noticed it today?