Why shias use to lie upon us?

Because they have been instructed to do that by their very own book.

I shall quote the detailed translation of sayings of Imam Sajjad (by Allah Sajjad is free from this filth attributed to him)

( إذا رأيتم أهل البدع والريب – غير الشيعي أو الشيعي المهتدي – فأظهروا البراءة منهم وأكثروا من سبهم والقول فيهم والوقيعة ، وباهتوهم – اي ابهتوهم بالكذب والبهتان – كي لا يطمعوا في الفساد في الإسلام ويحذرهم الناس )
[ تنبيه الخواطر ج 2 ص 162] .
[ وسائل الشيعة ج 11 ص 508] .
[ نهج الإنتصار ص 152] .

Imam Al-sajjad (as) said: If you see people of suspicion and innovation – other than shias or new shia – then show disownment from them and abuse them much, backbit them, make false accusations on them – that is, backbite them by attributing lies on them and make false accusations on them (‘Buhtaan’) …
[tanbiah al-khawatir v.2 p.162 – wasael al-shia v.11 p. 508 – Nahj al-intisaar p.152]

PS. Contributed by brother Umar ibn al-Khattab from awakening forums.