申万菱信是中国证券投资基金业协会的会员单位之一，而退会则意味着该公司选择退出该协会。针对申万菱信退会的原因，有以下几个可能的解释。

首先，申万菱信可能决定退会是因为其战略调整。作为一家投资管理公司，申万菱信可能在经营策略上发生了变化，需要调整自身在市场中的地位和角色。退会可以为公司提供更大的灵活性和自主权，以适应不断变化的市场环境。

Secondly, regulatory concerns might have driven Shenzhen Lingxin to withdraw its membership. The company could be facing issues with compliance or facing increased scrutiny from regulatory authorities. By resigning from the association, they may be attempting to address these concerns independently or seeking alternative regulatory solutions.

此外，申万菱信退会也可能是基于商业考虑。加入协会需要缴纳会费，并且需要遵守协会制定的行业规则和标准。然而，申万菱信可能认为这些成本和限制超出了他们得到的利益。退会可以帮助公司降低成本并更好地掌控自己的经营活动。

Furthermore, changes in industry dynamics could also be a contributing factor to their decision to withdraw from the association. The investment management landscape is constantly evolving and becoming more competitive. Shenzhen Lingxin might have decided that being a member of the association no longer provided them with a unique advantage or differentiated their services from their competitors.

最后，申万菱信可能也考虑到了其品牌形象和声誉。退会并不一定意味着公司有违规行为或负面事件发生。相反，它可能是公司为了保护自身形象和声誉而做出的战略性决策。退会可以排除与协会相关的任何负面联想，帮助公司维护良好的企业形象。

In conclusion, there could be various reasons for Shenzhen Lingxin to withdraw its membership from the China Securities Investment Fund Industry Association. These include strategic adjustments, regulatory concerns, business considerations, changes in industry dynamics, and protection of brand image and reputation. The specific reason behind their decision would require further investigation.