Most likely, your wheelchair is going into what is known as "turtle mode". This is a mode that limits speed for various reasons typically associated with safety.
For example, if the seat is tilted back far enough, turtle mode is automatically enabled.
There can be other reasons for turtle mode, such as motors overheating, brakes failing, and electronics malfunctioning.
I recently had this happen to me due to an electrical component failure. The ICS main controller failed and had to be replaced. Parts and labor were about $1,000. I am currently arguing with insurance regarding who gets to pay for it.
Steve