According to articles 97, 54 and 52 of the European Patent Convention, an examining division shall grant a patent only if it meets the requirements of the Convention. Article 52 EPC says that an invention shall be patentable if it is new. Article 54 EPC says that an invention is new if it does not form part of the state of the art.
Article 54 EPC requires a search in the EPO's massive documentation, using on-line tools to find relevant prior art. If insufficient time is available to search prior art, the likelyhood of not finding relevant prior art is significant, and the likelyhood of a patent being granted, because relevant prior art which should have been found has been missed for lack of time, increases.
If examiners are assessed on the basis of their output (the number of patents granted- or rejected) only, the risk of patents being granted, because relevant prior art was not found for lack of time, increases.
Also it is more time consuming to reject an application, because as an examiner you have to justify your decision to reject an application. If you decide to grant you also may have to justify your decision, but only to your colleagues in the examining division, not to the applicant. It seems to be a general principle of law that a decision by a public authority, which rejects a request, requires a justification, because in a way it adversely affects the person who makes the request. A decision not to reject does not adversely affect the person who makes the request, and therefore it is less important to provide a justification.
In the case of patents, you can imagine that it is much more easy to grant a patent, than to reject one. The examiner does not have to search so much for relevant prior art, just any prior art will do, and the justification is very simple
So, a policy to increase output (the number of patent applications processed by one examiner) will de facto imply an increase in the number of patents granted.
The Office likes to link the output of an individual examiner to the "Medium Term Business Plan", which is an estimated output plan over a number of years. If the number of applications processed as scheduled in the "Medium Term Business Plan" is increased, with an unchanged number of examiners, the load on an individual examiners increases.
By pushing for increased output the office accepts the risk of relevant prior art being missed, and also it accepts the risk of unjustly granted patents.
The Office may well deny that it intends to increase the number of granted patents.
It may claim that examiners may get an incentive to work more efficiently. De facto however, an increase in output implies less time in a prior art search, a higher risk in relevant prior art being missed, and a higher risk of an unjustified grant.
Lastly, it should be noted that the Office has a vested interest in patents being granted (not rejected) because granted patents bring (national) renewal fees, of which the office receives 50%.