Standard-setting organizations in general
First, who are the actors of this standardization process?
ECMA is an industrial consortium, that associates a few big companies (about 37 of them, e.g. Microsoft, Google, IBM,, etc) in the domain of information and communications technology and consumer electronics. The standards it establishes are optional:
It is not mandatory for Ecma members to implement any Ecma standard.
IEC is an international standard setting organisation, that associates entire countries (about 170) in the domain of electrical, electronic and information technologies. Each country is represented by a national committee that is representative of the national interests, in practice, public interests (government representatives/experts) and private interests (e.g. corporations).
ISO is an een larger standard setting organization, that associates countries without limitations on specific domaines.
The broader the interests represented in a standard setting organization, the more (and the longer) the discussions about the standard content. But also, the broader the influence of its adopted standards.
The effects of standardization
If ECMA produces a standard, other ECMA members may want to adopt it, for interoperability or efficiency purpose. But they don't have to. It's a non-binding private standard.
If after larger discussions on the base of an ECMA proposal, IEC or ISO comes to the conclusion that an important need is not well addressed, they might correct the standard. Once the ISO standard is out, all around the globe, ISO compliance will be expected. It might even be mandatory in some public context or in regulatory processes (see for example here about US, and here about EU where the international standards admitted in specifications are ISO, IEC and ITU as far as I understand).
Why would ECMA want to change?
When ECMA manages to push their standards more broadly via IEC or ISO, having their standard adopted by ISO is the paramount of success, since their privately decided standard made it through, world-wide. But if ISO changes something on the way, the ECMA initial standard is no longer aligned with the global reference standard. It would be too risky for ECMA members not to be compliant with the ISO standard they inspired. This is why ECMA may want to align back on ISO.
There is another reason as well: standards are like source code: they are version managed. So for the sake of sound version management, it might be interesting to bring the "ISO patch" to the trunc before working on a new version ;-)